(This is told from the character's girlfriend's perspective of course.)
Heat, that was what consumed your mind right now.
It had been about a half an hour since you arrived at the Zoo's Stables. You had always loved horses, which had attracted you first to the Stables and then to a handsome Arabian Stallion that made his home there. You had never been so close to a stallion before, and it was cool, at least until you realized what was happening to you.
But by then it was already too late as you had fallen onto all fours and your transformation into a mare had been completed. The stallion had mounted you while you were confused, prepared to take you as his own until the same attendant stopped him and whispered in your ear.
"Not just yet, I want you to turn your boyfriend into a stud. And now, you're a mare in heat, and "
You were disgusted by the idea of doing that, but you could feel the arousal start to grow in your new horse vagina. The arousal was starting to consume your mind as you did everything you could to alleviate it. You let out a whinny as you grinded your flanks and vagina against the walls of the stall you were in.
Not that it helped much, even after you came it just drove you crazy even more. You could smell the stallions around you, they were getting more and more aroused by your actions, and it was driving you wild. You needed to be mounted and bred, you wanted it more than anything and it consumed your mind.
Fuck, you had never felt like this as a human, and you knew that the bitch of an attendant wasn't going to give you relief unless you transformed your boyfriend. You neighed loudly as you came again, your ears barely registering the sound of someone coming into the stables and walking around followed by the click of your stall door opening.
When you finally turned your head you saw the familiar sight of your boyfriend standing just outside the stall. His transformation was already under way with his ears having turned into horse ears and migrating up to the top of his head, you could smell his scent changing as his pants became more tented.
He was going to be a stallion, you wanted him to be a stallion and take you like the mare you were. You didn't care that it was wrong, you just turned your back to him and presented yourself to the soon to be stallion.