What it says on the tin. The girl of the story is a female centaur, or perhaps some other form of horse-girl if you prefer. In other words, it's up for discussion, though I'd definitely prefer something more equine than not (so long as the character is intelligent, in a human sort of way). The race of the male lead, played by myself, is a little more flexible. Let me lay down a couple possible scenarios:
1.) Hunger of the Beast: The young equiness (just made that word up) is out foraging in woodlands far from home. Perhaps she is an exile of her people, or perhaps she's on a mission of some sort, traveling to some distant place on her own. In any event, she's in unfamiliar territory, and come nightfall she'll find that the seemingly docile forest is far more dangerous than she imagined. Ultimately, she's ambushed by a great beast of some sort, and in the ensuing struggle is disarmed and subdued, now to be victim to the creature's unexpected appetites. The 'beast' might be a powerful werewolf or similar were-creature, or might be something quite different, perhaps even a dragon or some other monster (open to requests).
2.) A Curious Offering: A nomadic, plain-dwelling tribe of centaurs or similar horse-people lives in fear of a dark warlord; a man of uncontested power who lives in an ancient, mountainside keep, from which he surveys and controls the surrounding lands as he sees fit. The tribe makes a pact with this man, giving him a great tribute in return for allowing them to forage and travel safely through his territory. Their gift to him is one of their own; a beautiful princess of their people, to take as his own wife or concubine. While many human men might have been shocked or even offended by the horse-peoples' proposal, this man is one who has lived for centuries and is eager to try any new pleasure that the world can offer. The warlord of this story would be a powerful vampire warrior, or perhaps a fallen angel who made himself a lord of the earth after his banishment from Heaven; again, the exact race of the male character is open to discussion based on the partner's preferences.
One thing I thought might be interesting about a vampire/centaur coupling is that there would be a sort of 'ancient Mongolian' dynamic, mimicking the idea that Mongolian warriors used to drink the blood and milk of their horses for sustenance.
Edit: Also, if you have an idea of your own, for a completely different scenario, then I'd love to hear it.