Arabella: Book One: In Her Lord's Stable by K.A. Halle
Arabella is a simple country lass, until the day she is captured into Lord Thayer's "coterie." Her life quickly becomes very very different. She is now in her lord's stable, sexually trained and erotically disciplined to become one of the Lord's mares. Then Arabella is informed that the highest honor in the land is to be hers: she is to be the virgin who is put to the "stallion," Prince Sajan, publicly at the kingdom's spring festival.
This book is a true classic in the erotic BDSM "ponygirl" genre. It is very very explicit, without being as harsh as many in the genre. If you liked Anne Rice's Beauty Series, you'll LOVE Arabella.