You see the lonely stallion in the field, notice his throbbing erection. Something about him enthralls you. You begin to walk over to him. You climb over the fence and into the field. He comes to you, but you immediately start to feel funny.
Your body begins to gain mass, and your clothes are already ripping at their seams. Your fingers begin to fuse together in front of your very eyes! You scream instinctively, but what comes out is a high-pitched whinny as your vocal chords have already changed. You shake off your shoes, and you feel the same happening to your feet! Your clothes cannot take the stress anymore, and rip apart simultaneously, leaving you naked. Your dick also changes, as it is now shrinking. You see it recede into your body... Leaving a horse's vagina! You feel your new sexual equipment rearrange itself.
Your back then rearranges itself as you feel as if you have been knocked on all fours. You feel a new appendage grow out of your tailbone- A tail! Your anus moves upward just under it, and puckers. Your new vagina moves itself to a more... Accessible... Region. A full coat of brown and white fur quickly encompasses you. Your head of hair begins to grow, becoming a beautiful mane. Your face grows painfully out into a muzzle, and reconstructs to better accommodate for life as a horse. Your hooves stiffen and become black. You feel teats begin to grow at your crotch. Your eyes darken over, completing your transformation.
That stallion seemed more excited than ever. And, for some reason, you wanted him. You wanted to bear his foals. You turn, and offer yourself to him. And he was more than happy to help. He heaves his body onto yours, and in one large thrust he penetrates into you. He thrusts again and again, rhythmically. With every thrust, you feel more and more like a horse. You feel your fluids seep out just as his potent semen rushes into you, the two liquids mixing together. He soon hops off again.