Bill's clothes were off in a hurry, and soon he and Megan were were in his bedroom. Because of her tail, Megan was on top, riding Bill hard to several orgasms before he blew his load into her. In the afterglow, Megan explained how she had gotten PEDs from her scientist friend, and immediately took a double dose to induce a change. The horse tail had been an unexpected side effect.
The next morning, Bill and Megan both had the urge to go for a run. This surprised Bill as Megan had never been a runner before. They started jogging then soon were running, racing around campus, Megan doing her best to keep up with Bill. He didn't go as fast as he could, but he was still astonished about her speed. The PEDs had worked wonders thanks to the horse hormones. Maybe he should have double-dosed from the start and he would have won one of his races?
Bill got steadily aroused throughout the run, his horse cock throbbing and straining against his running shorts. He began to notice a smell that aroused him further, which he soon realized was coming Megan. They exchanged glances, and without another word took the next turn heading back for Bill's apartment.
As soon as the door was shut and locked, Bill and Megan began aggressively tearing each other's clothes off. After some heavy making out and groping, Megan bent over the end of the couch. Her tail was up, inviting him to take her in her dripping horse cunt. Bill caressed Megan's ass, which seemed bigger and firmer than ever. Then he slid into her, thrusting away. Megan began making animalistic sounds, but when Bill slowed his pace out of concern she insisted he keep going. "Mate me, you big stallion!" she exclaimed.
Bill climaxed and Megan began orgasming. With each spasm of pleasure her ass seemed to push him, until the shoves caused him to pull out and fall on the floor. From there he watched in astonishment as Megan's ass stretched into a horse-like torso and finally two hind legs grew down. Megan finally collapsed to the floor herself from her expanded body size, her legs weak from the transformation. She was breathing heavily, exhausted and seemingly unaware of what had happened. Megan's equine body resembled a horse in shape, but not much more. Her hind legs were shaped like a horse's but still ended in human feet. The skin of those legs and all the new back part of her matched the rest of her body. She had become a more humanoid centaur - a humantaur.