I got one, well one with many, i don't think it has been done/seen before, but i love this idea: [Hyper, futa (if possible) growth/macro, excessive cum] a changeling growing from sex/pleasure, the growth piles up until climax. then the changeling shrinks back to normal, just to go at it again.

Wasp grunted and grinned as she felt her cock bottom out inside of the mare she’d been fucking. It always happened - she was a sizable specimen, with a “stallionhood” naturally peaking at just under twelve inches - but it was always a wonderful sensation.

“You like that, little pony?” Wasp hissed. Her wings buzzed madly behind her as her clit dripped sweet, sticky nectar onto the ground. “You like being filled up?”

The mare just moaned, her tight walls contracting around Wasp’s shaft. Wasp just bit her lip, her fangs poking over the bottom of her muzzle as she shuddered in pleasure. The rhythm of her thrusts lined up almost perfectly with the rhythm of the mare’s contractions, Wasp’s grunts echoing the pony’s cries of orgasm.

“Mm,” Wasp said. She closed her eyes and thrust in again. “But we can get you fuller.”

She pulled out, savoring the long, wet schlick of her massive cock. She focused her every sense on that feeling of tightness, of sex that seemed to encompass every fiber of her being. The sound of her fat, sloshing balls smacking against the mare’s ass filled her ears. She seized that sound and held tight, fantasizing about spilling that seed inside.

And Wasp began to grow.

As her pleasure mounted, so did her size. Slowly, inch by inch, her jagged “horn” began to rise above the ground even as her legs lengthened and her torso thickened. Her cock, too proud to be left behind, surged in size as well. Soon, it filled the pony mare so much that Wasp could feel the way her tight stomach was holding Wasp’s growing cock inside. She shifted her hips upward and groaned as she felt her cock-bulge sliding across the mare’s stomach.

The mare moaned, cumming again and again. Her own sweet juices ran down the length of Wasp’s cock as the changeling filled her up.

By now, Wasp would have been around the size of a large stallion had any been around to compare. Lucky for their egos, none were. Her throbbing, leaking, bulging cock was an incredible specimen, now stretching so far as to reach just beneath Wasp’s chest. The mare cried out in pleasure and some pain as Wasp rutted her mercilessly. Soon, Wasp was big enough that she actually had to crouch down a few inches in order to comfortably keep her cock inside of its sleeve.

That “sleeve” was becoming more and more so with each passing moment. As Wasp reached - and passed - the size of an alicorn, the mare beneath her groaned, writhing and twisting around the cock that’d speared her. But Wasp was too enamored with the pleasure her little cocksock was giving her to stop or slow down now. Plopping back onto her haunches, she took the little pony mare in both hooves and pulled her down.

The mare let out a wordless, gurgling cry as Wasp’s monstrous, throbbing cock bulged upward through her belly, stretching it so much that for a moment it seemed like there was more stomach than mare. Froth began to line the mare’s lips, but the emotional connection that Wasp had forged before beginning carried only raging, mindless bliss.

Wasp spread her legs wide as she hit another growth spurt. By now, she could have easily looked down on Princess Celestia, even sitting down. She must have been just over eleven feet tall standing up. Instead of doing numbers, though, she just kept pumping, literally “filling” the entirety of the mare up with her cock before pulling her back up, pausing, and thrusting her back down.

Her balls churned and sloshed, the tension at the base of her cock growing until it could be borne no more. With a cry, Wasp let her orgasm free, and each of her pony-sized balls set about to pouring its volume of cum into the poor mare’s belly. A second, smaller bulge sprouted in the mare’s stomach just above Wasp’s cock-tip, the sheer pressure and force of Wasp’s orgasm forcing the poor pony’s body to expand even further.

Wasp grunted and sweated as the mare’s mind went totally blank with pleasure. Gallons and gallons and gallons of cum flooded out from Wasp’s throbbing cock, filling up the mare’s belly and swelling it until she looked like nothing more than a cum-filled balloon, her belly jiggling like a full condom.

Her orgasm finally began to taper off, the river of cum settling into a stream, and finally shrinking into a trickle. Wasp just groaned. The dizziness set in quickly, and soon she found herself shrinking back down to size. That was the way it always was - the orgasm left her, and so did the size.

But the mare-balloon, her stomach fat and round with sloshing cum, didn’t get off so easily. She just lay there on her belly, groaning and panting with pleasure and exertion. Her hooves wiggled helplessly in the air, powerless to reach or touch the ground.

Wasp found herself panting as she hit normal changeling size again. Between her legs, though, her cock was already beginning to harden once more. She licked her lips and placed a tender hoof around her shaft, and looked up at the groaning mare above her.

Slowly, Wasp smiled. She could already feel her balls filling up again.