She opened a door that lead into the back room of the Stables through a door that you hadn't even seen while looking around. "Go on in, she's waiting for you," the attendant said with a smile before she gestured inside.
"Okay..." You said a little confused, why was she in here?
As you walked into the room the door slammed shut behind you and when you reached to open it there wasn't even a doorknob to let you in. You banged on the door shouting out. "What's going on? Let me out of here!"
"It won't do you any good, we're trapped in here," a familiar voice said behind you and you turned around and blinked in surprise at what you saw there.
Strapped to a table was the strangest thing you ever had seen. The face looked like your girlfriend's, but her chest had barreled out and her breasts had moved down between her legs. Or, what was now her hind legs, her arms and legs had become longer and more equine, her hands and feet were starting to fuse into hooves.
You could smell something coming from her, and felt your cock starting to strain at your pants and Jessica looked at you fearfully. She tried to cover herself up with her tail that had already finished forming.
"What's going on? How did this happen to you?" You asked as you looked at the strange mix of your girlfriend and a white and yellow mare who looked at you fearfully.
"It happened when I got here, that attendant she tricked me back here and I've been turning into a..." She said and she stopped in mid-sentence as her voice left her, her face starting to push out into a muzzle as she let out a whinny.
And that's when you looked in the mirror and froze. Your body was already starting to push out and your hair was forming along your neck into a mane. You felt an itch on your butt and saw a tail pushing out as your clothes strained, you were turning into a horse too just as your clothes broke apart revealing your new massive horse cock.
You looked back at the nearly fully transformed mare as you stroked yourself a little, your arousal rising a little as her tail went limp. She looked at you with resignation in her eyes as she realized that your fates were unavoidable.