I managed to hid my physical changes from my roommate. I let him know my girlfriend would be coming by the next day; when Megan arrived, he took the hint to go catch a movie. I had been for a run that afternoon so at least my muscular legs weren't aching for release. I found that while walking I had the odd compulsion to walk on the balls on my feet. After that surge of adrenaline I had to jerk off my horse cock in the shower, but had no doubt I could rise to the occasion later with Megan.
Megan was still walking awkwardly and, although her kiss was inviting, I knew she wasn't physically read for what we both wanted. We decided to actually Netflix and chill, although the movie we deliberately chose was The Black Stallion, which might as well have been porn.
As we held each other on the sofa, Megan stroked the hair on my legs, which had grown to resemble horse hair. However, she seemed a little agitated the whole time, and nervous. I supposed she was just vacillating over what to do - her desire to fuck fighting against her body's plea for rest. As the movie ended, she climbed onto me and kissed me hungrily and soon had her dress off over her head, with nothing underneath. She seemed sweatier than she should be, and were her legs more muscular than before? She practically ripped my shorts off as we made out on the couch. She seemed almost...aggressive.
"Megan, is something wrong?" I asked.
"I just wanna fuck," she replied. "I wanna fuck you so hard...uggggghhhh..."
That wasn't Megan's normal pillow talk.
Then she groaned again and grabbed her crotch. "Nooooo...what's happening to me?" she cried.
"What have you done Megan?" I asked.
"I thought if I took your horse drugs it would give me a horse pussy that could take the pounding," she admitted. "I found them when I used the bathroom right after I got here."
I understood. "Oh shit," I said. "Megan, those weren't just derived from horses. The main ingredient is horse testosterone."
Megan began to cry. "You mean...I'm...ugggghh...I can feel something...coming out of me..."
From between Megan's fingers came a penis, rapidly swelling into a horse cock, although not as big as mine.