You neighed loudly when you felt her hand grip around your shaft. "Oh perfect, we've had a distinct lack of stallions since we got Harry, this is perfect... we just have to get you ready to be mated with Jessie over there to finish both your transformations, which means your leftover human seed has to go."
She secured you and returned Jessie to her stall before kneeling down and gripping your new horse cock again. You neighed and whinnied as she started to jerk you off with one hand, rubbing your massive balls with the other. Your mind started to devolve into pure animalistic instinct as you started to thrust forward a little, throwing your head back as the horse mind seemed to overwhelm your human side briefly.
You let out a loud neigh as you felt your cum burst out of your cock and onto the floor, first little strands of human seed before several globs of horse seed fell to the floor. The woman just smiled and patted your side as your human mind started to assert itself again.
"Good boy, you're going to make a fine stud once I'm done with you," she said and took you and Jessie's reins and lead you into a back room where a phantom was waiting. She hooked Jessie up to it and stepped back.
You knew that if you did this there was no going back, you'd be a horse for the rest of your life and so would the mare, whoever she was. You slowly moved forward on instinct alone as a smell hit your nostrils, the mare was in heat, and your horse mind wanted to mate her.