You can't bring yourself to stop. The pressure has built too much and your too close to completion. You continue humping, this time consciously. Giving in to your lust causes the inevitable orgasm to come even quicker. The fruits of your lust come to fruition as the pressure reaches it's climax before finally it explodes into the mare you're mating with. The feeling is a sense of ecstasy unlike anything you've ever felt before. You keep at it draining every available ounce of yourself into her. After unloading every drop of yourself into the mare you finally get off of her. The realization of what you've done and what you've become starts settling in. You walk around on your four legs a bit, which is more natural to you then you're really comfortable with.
"Hmmm....that was wonderful, baby." you hear a voice say behind you. You turn to look and see the mare you were just mating with has turned back into her semi-equine form, the horse ears and the tail being the only things present from her formerly horse form. You snort in confusion. She smiles at your reaction.
"I was worried you weren't going to finish the job for a while there." she says and puts her hand on your nose and begins rubbing it "but you did. Felt good didn't it?" Your mind goes back to the climax just moments ago and how it was the most incredible thing you've ever felt. She smiles and seems to read your thoughts "Good. You're going to have many many many more like it before I'm done with you. Giving in to your lust meant giving yourself to me. Putting your fate in my hands. You're mine now, and I can keep you in that body as long as I want to." she says.
She pulls your nose closer to her face and touches cheek to the side of your long head. "Unless you go cheating on me with some regular run-of-the-mill horse. Then you're stuck in there forever. So be a good little lover." she says.