The fireworks were amazing, but they weren’t the most amazing. The most amazing is that she had a colt in her bedroom, and they were finally alone. She curiously felt at his colthood, while his own seeking hooves explored her soft nether lips. She didn’t realize how wet she would get, or how much it would make her feel... hungry. His strokes on her petals drove any doubts or hesitation from her mind, and it wasn’t long before she was stroking his stiffening shaft, as they lay alongside each other.

“This is so amazing!” the colt by the name of Peter Out said, a plain brown colt with a big heart and a curiosity to rival her own. She was Storefront, also known as Store, and he told the brunette filly with excitement, “I can’t believe our parents have no idea! I was so curious about this... you, here, and...”

“You’re not getting any bigger,” she said happily, nuzzling along his surprisingly thick, stiff lenght with a soft cheek, “That means you’re all ready to put it inside me.”

“Uh, y-yeah we could... do that,” he said, blushing hotly in the dim light. “You know what’ll... happen, right?”

“Oh I know, but, I was just thinking of trying it a little,” Store said, blushing herself, “J-just curious if you really can fit in there. I mean... look at this thing!”

“It’s kinda dark,” he said uncertainly.

“You know how big it gets!” she fumed chidingly, “How could that ever even fit in me?”

“I dunno...”

“W-well that’s why I wanna try it,” she said, “We don’t have to p-put a foal in my belly. I’m way too young for that to happen anyway.”

“I dunno, I’ve been cumming for a while... maybe you are old enough.”

With their vague mutual masturbation on hold, Store wanted to know everything. Sitting beside Peter, Store asked shyly, “What does it feel like to... cum?”

“It’s scary,” he admitted, holding her hoof, “It feels really good, but then I just have to keep squirting and squirting it. And if any of it got inside... y-you...” his hoof tightened around hers, “O-or any other filly, then poof you have a foal in your belly. I mean... stuff keeps coming out of me, that can make you pregnant. Just seeing it is like... wow I gotta be careful.”

“Can I see it?” she asked so hopefully, trying to imagine what it was like. She heard it was white, but that’s about it.

“Okay, but I can’t put it inside you if I already came,” he replies reluctantly.

“O-oh okay,” she said, ears drooping, “Maybe you should just put it in me then.”

“Sorry, it’s just when I um... cum,” he replied with a shrug, “My um... penis gets soft, and small again. Until I have more. Cum, I mean.”

“How long until you have more?” she asked.

“M-maybe a day,” he replied, “I never really p-pushed my limits.”

“Well how about...” Store said craftily, “You put it inside me, just to feel what it’s like, and then you can show me your cum.”

“I–I don’t know, I don’t wanna get any in you,” he said nervously, laying a hoof on her vulva again, “J-just if any splashes here, you might... um.”

“It’s okay,” she said relaxing as she ground her hips against his hoof, “I’ll be very careful.”

“Fine then, we can... try it just a little,” he said in less reluctance than she expected.

“Great!” Store told Peter excitedly, “So I was seeing a picture of ponies doing it, and he hugged the mare’s butt, so he could get it in. So that’s how we’re supposed to do it.”

“You want to show me it?” he asked hopefully, “I can’t really picture it. Hug your butt?”

“Yeah h-hug my butt,” she said, standing up on the bed, imagining herself in that mare’s place, except with her short little filly form, and the strong stallion was replaced with her dorky friend, whose penis had just grown in her hooves. “It’s too dark to look at the picture,” she told him, “And we can’t turn on a light or somepony could see us. I’ll show you after. I really want to try this out before our parents figure out we’re up here.”

“Y-yeah, they never let us even sleep together anymore,” he said a little resentfully. “I kinda want to try it, because it’s been a while since we got close to each other.”

“If you can fit inside my fillyhood, it’ll be closer than we ever were!” she crowed triumphantly, “So let’s get started already. Thinking about it’s making me all leaky.”

“Leaky?”

“I–I dunno, my fillyhood just gets like... I mean you felt it. All leaky and—”

“Slippery,” he prompted.

She nodded, “And stuff. I-it’s normal though. Just a fillyhood thing.”

“Have you ever tried putting anything into it?” he asked, “Just to test it out?”

She giggled at that, but was embarassed to say, “No, but I figure since we’re both foals your penis should fit perfect.”

“W-well I don’t have my cutie mark yet...” he said self consciously.

She nodded eagerly at that, saying, “Yeah, so I’m even bigger then! Your teeny little peeny won’t have any trouble fitting.”

“Teeny little peeny?” Peter asked with disapproval.

Store started to laugh, but then blushed. “If your penis is small, then my um... va...gina must be huge already!” she admitted, “Your penis really is big. I-I didn’t know I went that deep inside.”

“It’s probably just the right size to be inside your fillyhood,” he said dreamily, “I bet it’ll be amazing.”

“Well okay then let’s try it,” she said impatiently, fidgeting on the bed, “I want to have time to try it, and also to see your cum, so let’s hurry.”

“So, how do we do this?” he said standing beside her, equally fidgety from the way that his bobbing erection couldn’t reach her nether lips, at least not without Portals.

“The stallion is behind the mare, like this,” she said, turning to point her tail at him, “P-promise you won’t laugh,” she added, looking back at the shadowed colt.

“Um, okay?” he agreed tentatively.

Storefront lifted her tail. It felt... weird exposing herself for him. She was already so wet it left a sticky trail from her tail base. It wasn’t anything he hadn’t seen before, but somehow showing it to him, even in the darkness here, it was really cool.

“A-and now you just... climb on top of me,” she continued with determination, “Like you were walking over me, until your penis lines up.”

“Oh, I see!” he said wide-eyed, “So my belly is above you?”

“Y-yeah...”

“Wait, can you hold my weight?” he asked. The larger filly raised an eyebrow at the colt, “Right, sorry,” he said, walking forward. “S-so I just climb on you?”

“I guess so?” she replied, wiggling her tush. It did some sort of... wiggly in and out thing on its own, but she wasn’t sure he could see what it was doing in the darkness. A hesitant hoof lay on the cutie mark on her flank, then the other hoof planted on her back, and he started to climb up. “No not like—” she said, twisting her head to look at him, but he slid his hooves alongside her, his belly thumping down on her rump.

“I’m not gonna stand on you,” he said chidingly, “The whole point is... is getting my penis in the right place.”

“C-can you find it?” she asked nervously, her legs feeling stiff with the sensuous weight of the colt’s chest on her back, “Just... just try to find where you push it in. Probably where it’s the most wet.”

“Alright,” he said, poking her leg with his shaft, making Storefront shiver with eager indulgence. There was a colt touching his penis down there, just like mommy and daddy. She wondered what it was gonna feel like, as he poked and prodded at her down there. “...a little to the left, I think?” she asked the clueless colt.

“Thanks,” Peter said, poking more into that tingling center that made Store think so fondly of his shaft. Neither of them noticed their breathing getting heavier, or their faces getting flushed, their bodies preparing them to join together.

“Okay this has gotta—” he said, right as Store squealed in surprise as something in her seemed to wrap around him, before falling away. “You okay?” he asked, “I-it just feels good for me.”

“Oh, I just want you inside,” she said with a frustrated stomp, “Go right there again. I felt like I was um... wrapping around you, so maybe I just keep going in from there.”

She just kept going in from there.

“Woah, I can feel you going in!” Store said in shock, not sure what to do as she felt his penis sliding deeper and deeper.

“I... I wanna go in,” he said, pausing half embedded in her, “It feels... really good to go into you.”

“Rea—! Mmn...” she cut off as he pushed strongly forward, until his hips collided with her own. He felt so huge within her, it was hard to... to fit around him. Stretched deeply, Store started to realize that this is what a penis felt like when it was inside you. Peter’s penis.

“Store, you’re really hot in there,” Peter said, hugging her hips earnestly, “Like a really wet oven.”

“I–is that bad?” she asked nervously, looking over her shoulder at the colt buried in her snatch.

“No, it’s really good!” he replied.

“I can feel you too,” she said, squirming as her passage strained around his thick member. “Y-you’re really deep.”

“W-wow it’s like you’re squeezing me!” he said, grinding close against her, “I didn’t know you could do that!”

“I’m not!” she protested, “You’re just so big that it’s hard to fit!”

“Am I hurting you?” he asked worriedly.

She shook her head, “No, I just can’t... nnh... stop squeezing, because I-I’m not used to being... filled like this! Oh Peter, I c-can’t... believe your penis is inside me.”

“Just... if I came right now, it would all go into you,” he said huskily, “M-my penis is... right where it would put foals in. Store, I can feel your... your foal place!”

“Womb...” she mumbled, pretty sure that had to be the case. “It feels like you’re hitting my belly button,” she said, stroking a hoof down her own belly. He shivered when her hoof came into contact with his shaft. She could feel her own tender, tingling vulva just... wrapped around its base, and the rest of his penis was just... in her hips.

She spasmed around him just at the thought. “This is so dangerous,” she whimpered in barely restrained pleasure, “You’re a stallion ready to... to impregnant me with your penis!”

“And you’re a...” he said, as Store gasped in surprise at the feel of him sliding out of her, then back in again. Out and in, as she stood there stunned, Peter started humping against her. He was mating with her! “You’re a mare,” he panted, “I–I’m inside your vagina, and you have a womb. You want to be pregnated with my foal.”

“Mmn...” Store mumbled uncertainly, her whole pelvis radiating pleasure as the colt gone stallion hungrily spread her slick passage open again and again, slowly, experimentally, but not stopping. How did she let this happen?

“Store y-you know how my mommy,” he said with difficulty, “Got a big belly w-with a foal in her. That’s you! I-if I cum you’ll get round and... and achy and have a foal in you. You want that.”

“Maybe a little...” she said, her whole body tingling at the thought of a foal growing inside her, right up where Peter was sliding his penis. “Just go a little more...”

“I don’t wanna stop...” he said, hugging her tightly. “I-it could squirt all inside you.”

This was dangerous, and they should stop, she told herself. But she couldn’t stop thinking about a foal inside her, about his hot sliding penis, and his... his desire to squirt inside her. He was gonna do it, p-put his seed into her. All she had to do was let him. She couldn’t stop thinking about her stiff legs, her rear braced against him, filled with penis again and again. Her whole body was trying to make her into the perfect mommy, even her fillyhood was made to be filled with his penis.

He started thrusting harder.

“Sweet Celestia!” she exclaimed, at the aching, tingling stretching that Peter’s penis did to her. Her body felt hot and alive, as the colt gone stallion pounded her ass with increasing passion.

“It’s thick and gooey,” he purred, thrusting hard, “Just keeps shooting out of me. I’d be filling you with it!”

“Oh yesss,” she groaned, just the thought of that threatening to throw her over the precipice of a pleasure that never seemed to stop ramping upward. “It... comes out of... comes out of your penis’s tip, right?” she said breathlessly.

“Yeah, after I flare,” he replied curtly, grunting cutely as he pounded her rear. “Tip gets... really big... deep inside you.”

“Please... please do that!” she whined needily, “Get... big inside me, and... you’re so deep!”

“Store, I’ll cum!” he said desperately, “It’ll fill your place, and you’ll be a mommy!”

“Oh yes,” she moaned, feeling like she was gonna melt from the waist back, “Fill me up... gonna have a foal in my belly.”

They spoke no more, just grunting and thrusting together needily. Peter pushed Store against the pillows, but the important part, her fillyhood, remained up and full of his dick. Thinking of a foal in her belly and of his penis in her belly, Store fell into a trembling orgasm. She didn’t know what was happening, or what was up and what was down. Her tongue was out and she fought to breathe against these unbelievable waves of pleasure.

She felt him flare then, and it got so much bigger inside her, just like he said. She tried to imagine the gooey cum, tried to imagine her big mommy belly. It was just so incredible. She’d never wanted something more badly in her life!

Then his thrusts ceased as he drove himself hard against her. She was too wiped out by her orgasm to protest him stopping, and hot ropes of semen started to erupt inside her. He didn’t say they would be hot. Panting, staring forward, Store felt her vagina getting outright flooded with Peter’s seed. He held her tight and did it as deeply as possible, and Store wanted every drop to go inside her. She was probably gonna get pregnant now, but she could care less. The feel of that seed swelling inside her was just... worth everything else.

Her first time was incredible, way more than fireworks. Now that she knew how much she wanted it though, it was hard to resist. The second time felt even better. She could just stand there shoving back against him indulgently while he pumped more and more seed into her.

Every time they did it, she wanted a foal more and more. By the time her belly started growing larger with its treasured contents, instead of getting upset or telling their parents, he just fucked her into the ground. Then with a cunt full of his seed, she and Peter lay together, both stroking her new belly, wondering at its round tension, utterly infatuated with the idea that there was a foal in there.

They hid it, and protected it, and just let it get bigger and bigger, until one day Store felt her contractions increasing. She gave birth in the bushes, biting on a stick to muffle her cries. It hurt to push the foal out, way bigger than a penis, but it was also terribly indulgent when she gave into the urge to push.

She was a simple animal then, not worried about the world, or the foal’s future, thinking only about how to push the foal out and complete her transformation into a mother. It felt like an achievement, like she was getting away with something. She didn’t need to be pregnant anymore, because the contractions showed her that her foal was ready, and now she could just push them out out out. No worries about consequences, just her strange round belly that suddenly ached to contract again and again, her overstressed, stinging birth canal, and inside her, a foal. She couldn’t stop it. Nopony could stop it now. She was gonna be... a mother...!

The relief was palpable when it finally dropped out of her, her belly dropping dramatically in size. She licked the infant foal clean, chewing up the amniotic sac that contained it. It just felt right to do that, just like it felt right to contract, and push that foal out of her.

While she did that, Peter climbed up on her rump and started fucking her, eager to put another foal in her before they had to tell anypony about this. He had little trouble fucking her overstretched, goo soaked cunt, even as her womb continued to contract with the afterbirth neither of them knew about. While she panted in unadultrated lust, the foal climbed to its hooves, and unsteadily toddled over to her flank. There, it ignored the thrusting colt, and before she knew it, Storefront felt herself being nursed. It felt... really good, in a different way that did nothing to lessen the pleasure of her fucking.

She made him stop fucking her as soon as she felt his seed pumping inside. She growled that she wasn’t finished yet, and he backed off while she contracted strongly again in strained grunts. Did she have twins?!

Store looked at her vagina, and there was a long, red rope of flesh emerging from it. Following the rope, it went right to her own foal’s belly. That’s a umbili...cord? Her thoughts were robbed by another contraction, and as they continued something slippery and slimy slid out of her. “Oh... oh... oh... yes!” she shouted as it finally plopped out of her. A wave of relief went through Store’s body as a great burden had been lifted from her. She waited, but there were no more contractions.

“I did it...” she said, standing there limp maned and sweating, with a tiny foal nursing at her teat, “I’m a mommy now.”

“That was really gross,” Peter said, eyeing what had fallen out behind her. A grin split his mouth as he told her, “Your vagina is totally badass!”

“Thanks, I think?” she said dizzily.

Then they all got in huge trouble, but thankfully the rainbow factory was still operational, so Store’s foal got a full time job with benefits. Her second foal took a while though, because they made her take a pill to stop pregnancy, until she had come of age.