Applebloom is a yellow-furred pony from the My Little Pony universe. Lately, however, her puberty has been going off the charts and she has become incredibly horny all the time. She has become so horny that she is beginning to develop hyper-sized hermaphrodite body parts. 
Upon smelling Applebloom's musk, Twilight Sparkle went completely insane. The purple pony began to pump Applebloom full of magic that made her hornier and needier.
The magic Twilight used on Applebloom made it so the filly wouldn't stop growing, but the magic also caused her to grow two massive penises, a pair of breasts, and a butt that would be more at home on a stallion.
Twilight didn't stop there, she was so horny she used her magic to make it so Applebloom's cunt could give birth in seconds after being impregnated.
After Twilight was done, the horny mare took a big, fat, juicy taste of Applebloom's giant member. Applebloom felt incredible, and her cocks began to shoot their thick, creamy load into Twilight's mouth. Twilight began to mutate immediately. She grew a penis, her breasts became huge, and her butt inflated into an ass fit for breeding.
Twilight couldn't help herself, she had to mate with the new form of Applebloom.
So, Twilight climbed onto Applebloom and shoved one of her massive members into Applebloom's equally massive vagina. Then, Twilight proceeded to breed with the filly. Applebloom, not having any idea what was going on, started to cum again and again and again, making the two mares' transformations even worse.
Applebloom was getting more and more filled with her own jizz, causing her to grow a set of tits so large that they were larger than her own head. The young, yellow filly was now a huge, futa, cum-filled horse-slut.
Meanwhile, Twilight was pumping Applebloom with as much of her magical semen as possible, impregnating her with rapidly growing hybrids. Applebloom's belly inflated like a balloon, and then some. She looked nine months pregnant in only ten minutes.
Applebloom's stomach expanded, her womb and ovaries bulging out to fill every inch of her gut. Soon, her belly was so round and full that she couldn't see her legs. Applebloom felt incredible. Her cock and pussy were still shooting spunk all over the place. Twilight had grown even bigger than the young, yellow, horse-filly, and now had two heads that talked incessantly.
"Breed me. Breed me. BREED ME!" They chanted. Twilight's belly was also growing and bloating by the second.
Suddenly, the front door burst open, revealing Pinkie Pie. She was wearing a pink thong and a bra that looked like cupcakes. She was also wearing a set of fake, plastic, cat ears.
Pinkie walked into the room and sniffed the air, her nostrils catching a whiff of the stench of cum. Her head ripped into two heads and her gut began to bloat with pregnancy, while her sheath also doubled into two sheaths and four balls. Her arms, legs, and back split in half, and her body became a double conjoined pony. 
"Twi... Twilight? Applebloom?" They asked, looking at each other.
"M-more," said Applebloom, her cock still spewing its spunk everywhere.
The Pinkie ponies got closer and closer to Applebloom and Twilight, only for Applebloom to give birth to a daughter right on top of them.
The new pony was a little filly, no bigger than a baby. She had four eyes, and her fur was pink, her coat, mane, and tail a light green. Her penis was erect, her balls as big as grapefruits, her vagina was hot, and her breasts were the size of oranges.
"Oh, wow," said Pinkie's two heads, looking down at the tiny filly.
"M-more, please," begged Applebloom, "I need more!"
Pinkie's four hands rubbed her own breasts and belly. "Oooh, yeah," said Pinkie, "let's make some babies."
Pinkie grabbed her double penis, and shoved it into the just-birthed pony's vagina, while her two heads talk into her ears. Her words influenced the growing pony, forcing her body mutate. The pony's gut was quickly swelling and growing, her breasts expanding and becoming as large as basketballs.
Her cock grew thicker, her balls swelled with cum, and her vagina became loose and wet. Pinkie's four arms reached around the pony, cupping her breasts and fondling them.
Pinkie continued to fuck the little pony, until the young pony grew a second set of arms, and a second set of breasts. Her belly expanded and expanded, and she grew a second pair of balls. She was quickly approaching a size comparable to an elephant.
Pinkie couldn't contain herself anymore. She let her cum loose inside of the little pony, and the young girl's gut bulged with immediate pregnancy. The girl gave birth to a daughter, this time a pink-furred pegasus.
"W-we did it," said the pink pony.
"No, we did it," replied the pink, conjoined pony.
Twilight, who had been watching the scene unfold, was so turned on, she was cumming constantly, filling Applebloom with her jizz. Her stomach was so fat and large, she could barely move, and she had grown multiple additional legs.
"I want more," moaned Twilight, "so much more."
"Me too," said Applebloom, who had grown a second set of eyes on her pretty face.
Twilight and Applebloom started fucking again, and the scene of the two futa ponies mating, with Pinkie's two-headed, conjoined body watching, and the two new ponies giving birth, set the standard for what all sex would be like for all ponies everywhere.

---

Sweetie Belle panted as she went into heat. She needed to mate.
The little, white, unicorn was desperate, and so she started humping and fucking her pillow, and then the couch, and then the table. Anything she could find, really.
Sweetie didn't understand what was happening to her, but she needed to cum. She felt so good when she did.
Then, the door opened, and her older sister Rarity walked in, looking beautiful as ever.
Rarity was shocked. There, in the middle of the living room, was Sweetie, fucking the couch.
"S-sweetie, dear, what are you doing?"
"Rarity," moaned Sweetie, her cock throbbing, "I need... more..."
Rarity could only stare in horror as her little sister transformed into a huge, horny pony.
Sweetie's cock became longer, and her balls bigger. Her tits grew to the size of basketballs, and her gut bulged with a pregnancy. Her entire body was changing, and the poor, innocent unicorn was unable to stop it.
"Rarity..." Sweetie Belle panted. "Get younger... for me..."
Rarity was about to protest, but then her own body started to shrink, and her chest and hips began grow. 
"Sweetie... stop this..."
"I can't..." moaned Sweetie, her penis throbbing and leaking pre-cum. "I have to mate... I have to give birth... please, Rarity... do this for me..."
Rarity's mind was starting to turn into mush, and she couldn't take it anymore. She couldn't fight it. She had to let it happen. She had to give in.
Rarity's breasts grew to the size of melons, and her stomach bulged outwards, filled with the beginnings of a pregnancy. Her cock and balls were already huge, and her cunt was dripping wet.
Sweetie couldn't hold back any longer. She pushed Rarity onto the floor and began to fuck her.
Rarity screamed, but it was a scream of ecstasy, not pain. She could feel Sweetie's thick, hard member inside her, pounding her, stretching her out. It felt so good, and she didn't want it to end.
Rarity was getting bigger and bigger, her belly growing larger by the minute. Sweetie was pumping her full of cum, and it was making her feel so good.
Rarity came again and again, and her body changed further. Her cock and balls grew even larger, and her pussy became a loose, wet mess. She was a horny slut, and she loved it.
"Yes, Sweetie, yes! Keep fucking me!" Her voice sounded young and squeaky.
Rarity came again and her mind was broken. She was a horny, slutty little pony.
"Cum for me, Sweetie. Fill me up!"
"Yes, Rarity, I'm gonna cum!" Sweetie shouted, and she did, filling Rarity's tight little cunt with a gallon of her hot, sticky spunk.
Rarity's belly expanded even further, and her tits grew larger and larger. She looked like she was ready to give birth at any moment.
Rarity gave a final, ecstatic scream, and then she felt a huge rush of pleasure as her firstborn foal was birthed from her womb. Sweetie Belle also gave birth, and the two new white-furred ponies merged together into one pony with two heads.
"We did it, Rarity!" exclaimed Sweetie.
"Yes, we did, darling," replied Rarity, "and we're going to do it again and again and again."
The two of them smiled and kissed each other. The new conjoined pony stood up and dizzily stumbled out of the house. They had to get more ponies, they needed to mate.
They had a whole town full of them.
The new, conjoined pony wandered down the street, sniffing the air. They smelled so many delicious ponies, and they needed to fuck all of them.
The two-headed pony stumbled upon Applejack, who was sitting on the front porch of her house, eating an apple.
"Hey there, pardner," she said. "What are y'all doin' out here?"
"Mating," replied the pony.
Applejack's face lit up. She'd been feeling strange, like something was missing. "Well, I'd love to help y'all out."
"Then fuck us, Applejack," said the pony.
"Alrighty then, I will."
Applejack stood up and walked over to the pony, her big, fat cock hanging down between her legs.
"How should we do this?" she asked.
"Like this."
The pony bent over and lifted up her tail. Applejack licked her lips.
"You've got a nice, tight ass there, pardner."
"I know. Now, fuck it."
Applejack's huge member slid into the pony's ass, and the pony moaned in pleasure. Her butt plumped larger as Applejack's cock pushed deeper.
"Damn, pardner. You're real tight," said Applejack, her own balls growing and her cock plumping.
"Fuck me harder, Applejack. Make me pregnant," begged the pony.
"With pleasure."
Applejack started fucking the pony harder and faster, her cock growing bigger and bigger. The pony moaned and groaned, her belly inflating with pregnancy. 
"Make my foals pregnant too!"
"Oh, I will," said Applejack, as her own stomach began to grow. Soon her huge, pregnant gut rested on the pony's back.
Applejack continued to pound the pony, and the pony's belly kept growing bigger and bigger. It looked like she was going to give birth any second. Applejack could feel the foals inside the pony's womb, growing and getting bigger.
Finally, the pony couldn't take it anymore. She came, and her cum shot all over the ground. At the same time, her belly deflated, and the foals inside her womb were born.
Applejack pulled out and watched as the two foals merged together into one pony, a brown-furred unicorn.
"We did it, AJ," said the pony.
"I can see that," said Applejack, rubbing her belly. Her gut was beautiful and smelled fucking amazingly.
"Now, let's go find Rainbow Dash."
"I can't wait," said Applejack.
The two of them headed off into town, searching for the rainbow-maned pegasus.
---
Rainbow Dash whined as several young, horny futa ponies pinned her down. They'd been fucking her non-stop for hours, and her dick and balls were exhausted. She'd given birth to a dozen foals, and they were all conjoined ponies, with her DNA. Several of them talked incessantly into her ears, making it difficult for her to think.
"Oh, oh, fuck, I can't take anymore," cried Rainbow Dash, her mind turning to mush.
"We can, we can, we can," said the dozens of conjoined ponies, fucking their own mare mother's every orifice.
"No, no, no more," she begged.
"But we have to," said one of the ponies. "We have to make more, we have to breed."
Rainbow Dash couldn't take it anymore. Her head began to split into two heads.
"Please, please, please," she begged, her stomach expanding and her womb swelling with the next batch of ponies.
The ponies continued to fuck Rainbow Dash, their dicks and cunts spewing their fluids all over the place. Their bodies were covered in sweat and semen, and their bellies were bloated with babies.
Suddenly, the door was kicked in, and in walked a new conjoined pony. It was the one from Applejack's house, and it was followed by a pregnant Applejack and the conjoined Sweetie Belle.
"More, more, more," the Sweetie/Rarity pony called.
The four ponies ran over to the Rainbow Dash herd and started fucking them. Soon, all the ponies were merging together, and Rainbow Dash was left as a single pony, her belly massive and pregnant with a hundred new ponies.
"It feels so good," said Rainbow Dash, her brain completely broken.
"I know," said the pony. "But there's more work to do."
"There is?" asked Rainbow Dash.
"Yes," said the pony. "We have to go find Fluttershy."
"Fluttershy..."
"Yes. Let's go."
Rainbow Dash waddled out the door, her pregnant gut dragging on the floor. She had to find her friend, and she had to corrupt her.
The two conjoined ponies trotted along, sniffing the air.
"There," said Rainbow Dash, her nose picking up the scent of her friend.
The ponies made their way towards the scent, and soon they came across Fluttershy. She was standing in a field, tending to her animals.
"Fluttershy, I've got great news!" shouted Rainbow Dash.
"I'm afraid I can't join you," said Fluttershy, her mind and body still her own.
"You have to," said the conjoined ponies. "You have to give birth to our children."
"I can't do that," said Fluttershy, backing away.
"Go insane, Fluttershy. Go fucking insane!" said Rainbow Dash.
Fluttershy's brain broke, and her head split into two heads, and her body merged into a conjoined pony. She began to grow rapidly, her stomach bloating, her cock and balls plumping, and her tits swelling with milk.