A pony of your choice has made a habit of going to bars, picking up stallions, and offering them a little bet: if the other pony has the larger cock, she'll do anything they want. If he/she has the larger cock, he/she will take their entire length. The size he/she steals only adds to his/her erect length, allowing his/her penis to grow immensely, surprising the victim. Feat. dick stealing and degradation.

“Anything, huh?”

Twilight did her best Rarity impression, fluttering her eyelashes and lowering her chin just so. She even jutted out her lip a little bit. She was actually a little proud of how she got it to wobble. “Anything,” she said, then leaned forward until her lips were brushing against Soarin’s cheek. “Anything at all.”

It didn’t take a genius to interpret the way the Wonderbolt’s wings jerked up into the air. Twilight grinned to herself as she took in his quivering feathers. This was almost too easy.

“Compare my dick with a mare’s“ - Soarin’ snorted - “and bang the Princess of Friendship if I win? Hell, I’m definitely in!” He was definitely sloshed, from his unfocused eyes and shaking hoof. She almost felt kind of bad for him - but then again, if everything went as planned, even he’d enjoy the part he had to play.

Twilight grinned, leaned back on her stool, and raised an eyebrow. “Go on, then. Show me what you’ve got.”

Soarin’ glanced around the bar. He licked his lips and closed his eyes. “You asked for it, Twi.” With a grunt, he leaned his shoulders onto the bar and eased his hips a little further forward. Taking his tongue between his teeth, he began to stroke.

Twilight watched his growing erection with a vague sort of interest. It didn’t really matter what he did, really - she’d checked with Rainbow Dash last night, and while Soarin’s thirteen inches were certainly impressive for a stallion, she knew that she didn’t have anything to fear from this bet.

Soarin’ gave his flared head a few last pumps, groaning under his breath. He opened one eye and grinned at the sight of a sizable drop of precum dripping down his throbbing shaft. “Beat that, Your Highness,” he said cockily.

Twilight just chuckled. “Oh, I don’t know. I might…”

She loosened the invisibility spell she’d laid over her stallionhood before coming out tonight, smiling as Shining took stock of the impossibly wide shaft protruding from her crotch.

Her lips tightened. “…surprise you.”

Her smile only widened as she inched the spell downward. Unlike Soarin’, she didn’t need to masturbate to get hard - her erection had been throbbing every second she’d been in the bar, both for the anticipation of what she was about to do, and at the sight of so many cute asses. She eyed Soarin’s approvingly, giggling under her breath as her cock gave an appreciative throb. Soarin’s eyes just got even wider as the spell slipped down low enough to reveal her twin orange-sized orbs, dangling off the edge of the school and quickly filling the air with her musk.

“No way,” Soarin’ breathed. Twilight just smirked. Eight inches of her cock were now laid bare for all to see - now, ten. Twelve. Thirteen, and the bottom of the head was just beginning to come into view. Soarin’ had long since stopped sweating and had now simply turned a pale, clammy shade of whitish-blue. He’d lost and he knew it. Everypony knew it.

Twilight eased off the last few inches of the invisibility spell with relish: fifteen, sixteen, seventeen… When at least the nineteenth inch of her stallionhood came into view, she cancelled the spell with a flicker of her horn and allowed the room to just bask in the glow of her magnificent horsecock. It swayed gently, bobbing against her chest. It nearly reached her chin. Soarin’s own python - monstrous to most stallions - looked almost tiny in comparison.

“Come on, Twilight,” Soarin’ said quickly. He flinched as Twilight’s horn lit up with a bright, purplish glow. “You were just joking, right? I mean, you can’t seriously - “

“Oh, but I wasn’t joking.” Twilight licked her lips, staring hungrily at Soarin’s length. “A deal’s a deal, right? And, if my reading’s anything to go by, I should probably start collecting my winnings right - “

The same purplish glow sprang up around Soarin’s member. He moaned softly, his eyes rolling back up toward his forehead.

“ - now.”

Soarin’ and Twilight moaned in unison as the spell sapped each thick, veined inch of Soarin’s stallionhood, only to add it onto Twilight’s just as quickly. Soarin’s shrunk from thirteen to twelve, then hit eleven just a moment later. Soarin’ could only stare in a dulled sort of shock. He hadn’t been below a foot in length since he’d been a teen. And now? He let out a choked-off sob.

But Twilight was just getting warmed up. Her now-twenty-one-inch cock was still growing, and Soarin’s still shrinking. Twenty one inches became two feet, and eleven became eight. Soarin’ was now just above average for a stallion, and if Twilight had let it stop there, he would have been almost grateful. But one glance at the hungry look in her eyes proved that to be a false dream.

“Mm.” Twilight groaned pleasurably under her breath as her cock surged another two inches in length, Soarin’s shrinking down to six. The entire bar was watching them now, frozen into a mixture of muted horror and arousal. “I just love being so big.” She gave him a wink, her tongue flitting between her lips - and darting against the side of her enormous flared head, which now reached easily up to her muzzle. “Not that you’d know anything about that, you little colt.”

“Not a colt,” Soarin’ grunted, but that claim was becoming harder and harder to uphold. Six inches had become four, and his once proud, tennis-ball sized nuts had shrunk to the size of walnuts. By contrast, Twilight’s ballsack looked positively weighed down with its pair of canteloupes, each one sloshing and churning audibly with seed. She licked her lips, groaning as a tablespoon of pre spurted from the tip. She smirked even wider as the pre splattered on Soarin’s forehead to begin dripping down his muzzle, its overpowering musk reminding him of all that he’d lost.

“Stop,” Soarin’ whimpered. Two inches now. Twilight’s monstrous shaft was simply enormous - the biggest specimen that anypony had ever seen. Soarin’ felt so small, felt so tiny next to that towering pillar of stallionhood. Thirty inches of massive, throbbing maremeat would have put his original length to shame. He would’ve looked like a child beside it. And now…

He let out a sob as his sock shrunk to a measly half-inch in length, the glow fading around his “fun-sized” balls. Now, even a foal would have outmatched him. Even a foal would have been larger than he. Even a foal -

Twilight curled one hoof around her massive, bobbing member and held it fast to her chest. The tip just barely reached the base of her horn now, and the shaft itself must have been five inches across. She ran her tongue up the side, slowly, savoring the attention, as she happily slurped up a line of precum steadily dripping down the side. She searched the crowd, looking for a familiar face. Who to ask, who to ask.

“So,” she said casually, looking Big Macintosh right in the eyes. “Who’s next?”