Development App

Author's Note: Requested by Shironomi.

Holly wasn't like most centaurs. Most in her age group stood head and shoulders above her, almost torsos above human girls, with some that nearly doubled others. Her mother was one such centaur, several feet of legs that supported a voluptuous torso with tits half the size of Holly's whole body. All that said, she didn't show much of any jealousy about it.

The fact she wasn't too big for most humans played a part. Centaurs had a problem after puberty wreaked its havoc, that being sex with humans was nigh-impossible with three and a half foot dicks, as was the average. For that reason, possibly the only one, Holly was popular with her classmates. She couldn't offer rides like her kin, but she could ride others. 'Could' being the core word. The centaur was too shy about sex with anyone but her closest friend, Jane.

She could've grown into something truly spectacular, though. The potential was there, that's what saddened Jane the most. Having met Holly's parents, both equally massive even by centaur standards, it was a wonder how their child turned out the way she did, though her shyness made sense. Her parents were almost obnoxious in how 'out' they were about sex, which had led to may awkward encounters. Both for Jane and Holly.

That was when Jane made a discovery that changed her whole life's focus. Go figure, it was an app, not a game or some weird health care thing, though perhaps it was more of a mash-up of the two. The first page was just a typical home screen, a few bars of 'progress' and daily objectives. The real kicker was the focus; sex. The bars were dicks, the weird stamina gauge was a pair of testes, and she was pretty sure all the buttons were just nipples or clits. On another screen was a body.

At first, it resembled hers eerily well. Like something had taken 3d scans of her from every angle, though she hadn't a clue how that was possible without her knowledge, but there were settings. One changed it into the shape of her dearest friend; Holly. She left it alone for a few days, until after she and Holly slept together again.

It was an ongoing arrangement between them. Both had needs, Jane was flexible enough to handle a two-foot horse cock, and Holly was a futa centaur, of which Jane had yet to meet one without an insane libido. They'd been friends since junior high, which turned intimate after a chance encounter in the restrooms. Neither wanted a relationship, so it was a friends with benefit scenario, which suited both just fine. No strings. No commitments. Just good fun.

Which only got better as Jane dove into the app. The 'quests' were for all manner of sexual acts, from some simple like oral, to being edged ten times in a row, and even living a full day in bondage or with a full blown erection. Which Jane made every effort to engage in. Most were easy, they already did them on a regular basis, but it took some convincing for Holly to go a full day with vibrators inside her. After that, however, it wasn't so hard.

The app claimed responsibility for that. A menu came out that showed Holly's stats, from her sizes all the way to her psyche, represented as a meter going from 'lewd' to 'reasonable'. It started deep into 'reasonable', but each successive act tilted her more in the opposite end. Subtle things changed about her wardrobe, such as her shirts dipping lower, or the slight swing to her gait now, like she was trying to show off for the masses. And she had more every day.

Completing those quests earned points, which Jane kept stored, but it gave automatic bonuses anyway. She'd first discovered it a year ago, saving points for when the urge really struck, and basked in the way Holly developed. The centaur once stood just a few inches over Jane, now she towered as she was meant to, with over a foot added, though her assets stole the show and encore. It did result in some challenges.

Sex wasn't an option for starters. Flexible as she was, Jane couldn't handle her friend's new size. From two feet, all the way to four and a half fucking feet of pure womb wrecking cock, and her breasts had exploded just as impressively. Not to mention her ass. Odd as it was to see a horse body with human curves, it was undeniably sexy to watch her cheeks jiggle and sway with the clopping of her hooves.

"Are you sure about this?" Holly asked. Jane had spent the night, using an impressive arsenal of skill and toys to get her friend off over and over, until the centaur's cock looked ready to explode. Now, they were ready for school, with one minor addition. The 'erection' quest offered the biggest rewards and was simple to accomplish, but Holly never went with it until then. It was simple; maintain erection throughout the day, for every hour done she got points.

"Don't worry, no one can see," Jane assured her, adjusting the carapace so her words were true. Though centaurs were exempt from wearing coverings in public, at least on their horse body, walking around with a throbbing erection didn't sit well.

"Okay."

"Is that disappointment I hear?" Jane teased.

"N-no! It's just... maybe..." Holly's cheeks flushed scarlet, which she hid under her hair and galloped from the room. Her carapace fluttered, revealed her juicy balls and a flash of silver from the ring Jan put around her. The human chuckled to herself, wondering if today would be the day she finally went all out.

With Holly's new physique, it was possible to ride her in the proper fashion and still get off. Fat and cock weren't the only things added, muscle piled on in equal measure, which rippled under her hide as she trotted along. The constant rocking motion against Jane's womanhood, dampening her panties along the way to school. It was the only reason to wear skirts, other than in the summer. Ease of access became a priority when she was constantly horny.

Jane orgasmed as they approached the school, panting against her friends strong back. She traced the muscles from the base to her shoulders, enjoying their supple rigidity, and dismounted with a sigh.

"You're sure no one can see?" Holly asked.

"I'm sure."

"You came on me again, didn't you?"

"Was it that obvious?" Jane asked, "Couldn't help myself. The way you move is so perfect."

"It's fine," Holly looked down at herself and sighed.

"What's up?"

"I wish we could do it properly, you know?" Holly said, frowning at the slight bump of her cock as it peeked out her front legs.

"I know, but such is life," Jane said. Everything the app changed, whether small or massive, no one noticed. Even Holly herself, or her own mother never brought up how the centaur had grown exponentially. She couldn't fathom how the app did it, but it saved her an explanation. Far as anyone else knew, it was just the Holly grew. One day she was a petite centaur, the next she was like the rest with a bigger than average cock.

Though people accepted it as fact, things hadn't changed about her possessions. The school catered to its centaur students with desks tailored to each of them, as such Holly's was small, not much different to a normal person's. Now she forced all seven feet of her new frame into it, succeeding to comical effect. Jane adored the sight, though. It only highlighted just how massive her friend was getting.

And hinted at how big she had yet to be.

Joy danced through Jane's body as she watched the quest metre filling. With Holly's erection still engorged, it reached a new milestone every few minutes, adding points and passive buffs to the centaur's stats. The changes were slight, but there if the right eyes watched for them. Holly filled out more and more, her carapace lifting at the front as her cock grew. The four feet was just the shaft itself, excluding the sheath - which measured close to three-foot on its own.

The head inched out from her forelegs, eager to be seen by their classmates. Holly ignored it, eyes dead ahead, though Jane had long since forgone any subtlety as she stared at her friend. She couldn't take her eyes off it, not even after changing classes or when she was called on for answers, her eyes refused to leave Holly. Especially when her shirt tore down the front.

It held on by several threads, just concealing her nipples, but that wouldn't last. Jane licked her lips and glanced at the time; one hour until the day was finished. Her friend was already huge, crammed into a tiny desk space in the middle of everyone else, and what better place to really put the app to use. Jane fished her phone out in secret, so it wouldn't be confiscated; the last thing she wanted was the teacher putting the points in the wrong places.

Although, there wasn't really any wrong choice. Size - which opened into sub options for Holly's specific parts, Hybridization, Orgasm, Virility, Lactation, and Duplicity. She had over 200 points to use, with the limit on each being 100. Most already had a fifth used, but she couldn't hope to fill them all in. Jane pouted over her lack of points, though she could always get more. And the growth might lead to completing a few quests in the meantime.

"Okay, Holly. Let's see what you can turn into. Better start with something simple," Jane mumbled and put ten points into the overall size option. A second later, the final threads of Holly's shirt gave out, held in place by the centaur's magnificent nipples alone. Jane wondered if she could even wrap her hand around one.

That wasn't all. Holly's carapace suddenly became too short around her legs, riding up to reveal her overstuffed balls and the underbelly of her cock, each of which bloated at their own pace. Muscle also added to her equine half, thickening her legs to resemble prized stallion racer. Her breasts weren't excluded, blossoming even further into basketballs. Jane licked her lips and pushed a discreet hand between her thighs.

If that was just overall size, with the points being diluted between all the others, then she needed to see just what the specific options would do. Eyes locked to Holly's luscious tits, she added a single point, and nearly choked as they instantly swelled another few inches. Just one point did that. Jane pushed a few fingers inside herself, dreaming of what she could do. Why dream? She thought and giggled at her options.

Which one, which one, she wondered with her fingers working at her pussy. Holly shifted her weight, highlighting the way her bloated sack lifted the carapace, the spheres massive and straining the flesh to a sheen. The girl sat behind her acted oblivious to them, like a giant pair of balls weren't right in front of her. Maybe if they were bigger she'd enjoy them more?

Jane knew it was just her own want, but she didn't care. Her finger found the 'balls' option and piled on the points, twenty in total. That should be interesting.

Unlike Holly's boobs, they took it slow. Less than an inch gained every few seconds, burgeoning from her hind legs and crawling toward the girl behind her, who barely glanced up at the giant spheres headed her way. They reached her desk, pushing against and around it, even taking up the gaps at the sides and encroaching on other students. Jane was sat beside her friend, safe from the expanding nuts.

"Sorry about this, Megan," Holly said to the girl behind her, who was still trying to write down notes despite her desk now being occupied by a set of gigantic balls.

"Don't worry about it," Megan said and gave the centaur's sack a firm pat. Her fingers lingered, before she snapped them away, as if realising what she was doing. Must be her musk, Jane thought and sniffed the air to catch a whiff of it. Just the smell made her cunt shiver in want. The growth slowed and stopped, leaving Holly's balls grossly disproportionate to the rest of her.

"Let's fix that," Jane giggled and put a few points into everything else.

"Not again," Holly sighed, like her growing was any other inconvenience, even as her cock inched toward the girl in front, rising to loom over her head. The centaur's ears flickered and a small whinny of pleasure slipped from her mouth. The girl ahead of her had moved aside, letting the behemoth prick lay on the desk. She even rested her arm on the thing and ignored the out pouring of pre-cum. At that size, Holly's carapace was like a child's blanket over her body and hid next to nothing of her endowments. The shirt held onto her nipples for dear life.

Thumb poised to deal with that, Jane stopped as something caught her eye; Holly's sheath. Nothing new by itself, aside from its current size, but the vague shape of a small cock peeking out had her interest, as did the curious protrusions along the originals length. She looked to the Hybridization and Duplicity options and realised just what they meant.

"Oh, this is too good," Jane licked her lips, fingering herself to the frantic rhythm of her thumb tapping between the two. Fifty points between them, still leaving her with over a hundred left. She'd ruminate over what to use them on while she enjoyed the ludicrously decadent show.

The protrusions turned to thick, flexible spines along the leathery shaft. Each had a tiny slit up front, from which a dense, clear liquid drooled out, though the girl getting dripped on didn't seem to mind. Nearer the base, where a second cock was growing, a massive, oblong sphere was growing. Further and further, until it had to be double the flared head's mass. The copy also had the same traits growing in tandem with it. Jane bit her lip to hold back a moan as she spied a third cock on the other side.

Silently, the students in their paths made room for the cocks to sit on their desks. One girl even took a selfie with it. The spines and knot weren't all either, as Holly's shirt was pushed up and off by a second and third set of tits, each the size of their original. The second they finished growing, each nipple split apart once, then twice, leaving her with four luscious teats per breast. Milk dripped from them all.

Holly's tongue lolled out as she touched her new body parts, though she might not think of them as such. It had also changed, becoming long and flat like a canines, with a rougher texture over the top. Her equine traits also shifted a little more throughout. Her coat of fur became furrier and softer to the touch, like a cats but with the colour of a Siberian Husky, and her tail waved side to side in leisurely pleasure. Next came her hooves, which flattened and expanded into paws.

Were it not for her horse ears and the distinct shape of her cocks, Jane wouldn't have guessed her friend was ever a traditional centaur. What could she be called now? Questions for later, Jane decided and slowed her masturbation to browse her options. She glanced at Holly's balls, which to her lack of surprise, had also duplicated into four. With so many tankards and so much room, her current production not be enough.

That in mind, Jane dumped half her remaining points into Orgasm and Virility. No outward changes happened immediately, then she heard the gurgling. She followed the sound as it travelled up each shaft, then all three girls catering to Holly's dicks yelped as they were doused in pre-cum. Another gurgle, this one deeper and heavier, like something bigger was coming. The pre-cum changed texture, thickening further and further. No, that wasn't it. They were less fluid than before, now comprised of giant spermatozoa.

Jane couldn't help herself and crammed her fist inside to trigger a giant climax. Amidst the waves of carnal delight, she felt nothing wriggling around her feet and glanced under her desk, where a sea of sperm were poking at her. Almost the entire floor was covered, and Holly's production wasn't slowing down either. The students nearest to her took care not to touch her, fearing they'd cause more to appear, which was easier said than done.

Especially when Jane still had several dozen points to spend. Where, oh where to put them, she mused in her afterglow, then rolled jumped at the sudden pinging from her phone. Several quests had been fulfilled all at once including, to her shock, 'First Impregnation', along with the second and third, all the way to the tenth. Jane glanced around to see covert hands scooping up the mess of giant sperm and shoving it into their panties. Guess people are more perverted than they let on.

The series of completions meant more passive growth. She pumped her fist once more, slowly as she was still sensitive from the last orgasm, and committed every centimetre of her friend's growth to memory. With so many quests done, it was an impressive burst. She guessed Holly was encroaching on eight feet before, but now she had to be well above that, and the rest of her wouldn't be left out either. The barbs on her cocks resembled their own, miniature phalli now.

"What I wouldn't give to get them inside me," Jane groaned, reminiscing on the feel of her friend's old, two footer and how it stretched her so good. She'd figure something out one day. The growth was more than enough to get her off though. That, and Holly's increasingly lurid behaviour. She wasn't even bothering to apologise anymore, even as her dicks took up more desks ahead of her, and her balls forced Megan to climb on top or be crushed. Milk poured from her plethora of nipples and down her body now.

Finally the passive growth stopped, but Jane wouldn't let it end there. Her points were replenished, well over a hundred once more, and she still had half an hour left before school ended. If her friend could cover the floor, then what could she do with a little more? Jane slammed her thumb on overall size, then switched between all the specific choices until every point was used up.

Without need for her hand up top, it joined the other at her pussy and fiddled with her throbbing clit. Now all she needed to do was sit back, relax and... fuck it. Jane pocketed her phone, just in case, and stood up. No one looked at her, too busy with concern over Holly's gargling balls and cocks once more. The human, now dainty compared to her enormous friend, climbed atop the central cock.

Holly focused on her, though the centaur never stopped groping herself. A wave of milk doused Jane, but she just licked up she could and scooted closer to her friend's torso, even as it grew bigger before her very eyes. Beneath her, Holly's cocks swelled and pulsated with the increased flow of pre-cum, which really was just massive sperm at that point. They'd grown as well, easily a the size of an adult cat and only growing.

"I think this was a bad idea," Holly said.

"What was?" Jane asked and nuzzled into a breast bigger than her whole body. She found a nipple and latched on, guzzling down the sweetest milk she'd ever had. More like a cream, she thought and wondered if it was high in fat or sugars, then decided she didn't care. Most of what she consumed went to her ass anyway, and she doubted Holly would mind that.

"The cock ring," Holly moaned and cradled her head, gently as her hand could easily envelop it.

"It's still on?" Jane asked, wide-eyed as she glanced back. Had it also duplicated for the others cocks? If so, then the current production was nowhere near Holly's true potential. The prospect was terrifying... and so fucking hot! How long could the ring, or rings, last for? She couldn't wait to find out.

"Yeah. I knew I shouldn't have let you do it to all three," Holly said, then realised her friend wasn't drinking from her anymore, "That doesn't mean you should stop."

"Oh, sorry," Jane said and returned to drinking her fill, even as the nipple engorged down her throat. How many people could say they deepthroated a nipple, she mused and did what came naturally, bobbing her head back and forth like a pro. Her pussy was sopping wet just from the experience, and feeling her friend's endowment filling out against it. Jane glanced down at it and saw that her body was dwarfed by the cock.