The jenny in the first stall is very receptive to your advances - it seems odd, even a little twisted to be mounting a donkey, but you are enough jackass that it doesn't really bother you. You find yourself moving through a courtship ritual that includes catching her scent, rubbing against her, mutual braying, and finally, the actual sex.
Her black opening is slick and warm, clamping around your shaft as you push deep into her. Each thrust seems to make you swell inside her even more, and you bray in pleasure as your whole body throbs.
Release is incredible - it feels like you pump a gallon of semen into her, and you shudder as she milks the last squirt from your enormous testicles. Panting, you pull free of her after a few minutes.
The sensation of getting larger was no illusion - from the waist down, you are now full-sized jackass. You can walk on your hooves without problem, though you have a slight hop-step for a walk. You are fully furred, and your head is now fully jackass as well. If you leaned over and let your thick, black three-fingered hands touch the floor, you would look like a normal animal.
The scientist, who now is shorter than you, points to the next stall. "I have a lot more in need of service."
You can feel your balls getting heavy again as they refill - mating is obviously a prime function of this new body. But if you give in to the building desire, you'll lose your arms and hands.
Forcing yourself away from the stalls, you find a notebook at pen and print a request in big block letters: