Following up from the Flutterbro and Muscle Pie...

After a long, hard day of flying with the Wonderbolts, Rainbow Dash got her water bottle out of her locker. Well, it looked just like her water bottle. She never noticed the gender sign on, just immediately taking a big swig from it to cool down. The effects were immediate.

The effects were also fast. A far more active and physically fit pony than her friends, Rainbow took no time to swell on up, hitting a strong, buff frame. Her uniform slowly shrank and shrank, splitting into a shirt and tight shorts of sorts. She didn't notice a thing about.

All she noticed was that she felt good and cool, cooler in more ways than one. She needed more and fast! She sprayed the seemingly infinite water bottle into her mane, swelling her... him faster.

In only a matter of minutes, a new, himbo stallion stood proudly within the Wonderbolts locker room. His uniform was reduced to some stretchy, tight, gym shorts, but surely he could get a new one later. Rainbow Blitz was a little surprised by this turn, but more than happily accepted it. He was stronger, denser, fitter, and way more than 20% cooler now. He could seriously love this.

Heh, if only his bestie and rival, Applejack, could see him now! In fact, why not fly down and visit Ponyville right now? This was important after all.

Soon, the locker room was empty, a rainbow flying off for home in the distance. Meanwhile, their water bottle was left alone on a bench, out in the open and available for anyone to use......