Twilight Sparkle was born a futa, and thought her puberty was over. Then her cock and balls start growing! And will not stop! I know this seems a bit of a tall order, but can you make it work? (Anthro or pony is your choice.) If anthro, titfuck?

At first, Twilight hadn’t been entirely sure what was happening.

It had started out as just a normal masturbation session - door closed and locked, lights turned out low, and Spike safely (and securely) in bed. She’d gotten out an old magazine she’d kept around since first discovering what an “orgasm” was. Its title was Lewd Mares, and Twilight’s absolute favorite was the centerfold on page 17.

She bit her lip, her hoof slowly running up and down the length of her cock. It wasn’t unknown for a mare to be born with an additional “tool,” but it was certainly uncommon, and more than occasionally looked down on. But Twilight had always reveled in her status. She loved the fact that she could explore two sexualities, instead of just one. And, while her member might have just been a tiny bit below average for a stallion her age, the pleasure it gave her more than made up for it.

So Twilight slowly ran her hoof up and down her length. Between her legs, she could feel her balls heavy and full with her seed. It’d been a few days since she’d last had any kind of release - too busy with studying, she’d guess - and so it she felt more pent-up than she had in months. Idly, she wondered how big of a cum splatter she’d leave tonight. She made a mental note to measure it later.

But as her hoof traced small circles around her tip, Twilight slowly realized that her other hoof, which she’d settled at the base, was having some difficulty reaching all the way around her shaft. In fact, her cock felt…thicker, somehow. Weightier.

She glanced down and stared. Twilight Sparkle had never been particularly well-endowed, lingering at somewhere around five or six inches for most of her post-adolescence. Yet the throbbing length currently staring her in the face had to have been eight inches, at the very least! She reached around and gave her shaft a soft squeeze, and gasped at the jolt of pleasure that tingled through her spine.

This should not have been possible. As far as Twilight knew, she hadn’t taken any kind of virility potion lately. But there was no denying the new size of her member - and there was certainly no denying the two extra inches it gained as she watched, her thick, throbbing cock reaching a full foot in length and just over two inches in width.

Her balls, she noted, were swelling too. With a look of quiet awe on her face, Twilight cupped one of them in a hoof. It was heavy - nearly the size of a tennis ball - and she could almost swear that she could feel the cum churning inside. Then, right before her eyes, her package began to grow again.

Twilight moaned quietly, her hooves going right to her flared head as her dick left a foot behind and began the march to two. Thirteen inches came and went, but for a moment, she thought it would stay at fifteen. It was nearly half as thick around as one of her legs, and around half as long. If she’d been standing, it would have dangled noticeably between her legs, her fat, cum-filled balls dangling quietly behind it. As it was, though, while hard, it was more than long enough to bob gently against her chest.

Slowly, she reached down, muzzle held slightly open. The instant her tongue twitched against her cockslit, it felt like she was in heaven. She gave it another flick and groaned at the resulting flare of pleasure. She pulled her head away, leaving a fine thread of saliva going from her lips to her tip.

Her cock seemed to have finally petered out in its growth at eighteen inches - not quite two feet (a part of Twilight’s mind screamed incoherently at the very idea of “not quite” two feet), but more than enough for her to enjoy. Bobbing a few inches below her chin, and looking thicker and tastier than anything she’d ever cared to taste, it was the most attractive thing she could think of.

Twilight licked her lips, all thoughts but sex banished from her mind. Her eyes flicked back over to the centerfold on page 17 of Lewd Mares. She wondered how much of her cock that mare could take.

Or, she wondered, how much she could take. With one hoof rubbing her heavy, churning balls and another steadying the base of her enormous shaft, Twilight opened her mouth and slowly leaned down.