You see the lonely mare in the field, and you are entranced by it. And it is entranced by you. You begin to walk over to it. You climb over the fence and into the field. She 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 neigh 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 demands attention, as it is now completely erect. But it is also non-human. It is pitch black, incredibly long and incredibly thick. It looks bestial.
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 scrotum dangles down just under. A coat of black fur grows quickly over you. Your head of hair begins to grow, becoming a 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. All of a sudden, semen squirts out of your still-erect cock, and you feel your testes fill with potent horse sperm. Your eyes darken over, completing your transformation.
The mare nuzzles against you. You can smell her female scents... You want to be on top of her. She poses with her loins facing you, offerring you her sex. And you oblige. You lift yourself onto your hind legs, andmount her. It takes you a few tries, but you plunge your penis into her. You thrust, in and out. In and out. You feel the pleasure in your groin accumulating. She obviously felt the pleasure too, as you feel her vaginal juices cover your cock. You can't hold on any longer, and you release a tonne of your warm seed into her eager womb.