Horsemistress Saga Series by Toby Bishop

In the Duchy of Oc, the most precious of creatures are the winged horses blessed by the goddess Kalla. When one is born, it is immediately taken to the Academy of the Air to be trained and watched over. But when a spirited peasant girl bonds with a winged horse of her own, the Academy gets more than it bargained for.

Since bonding with a winged foal by accident, Larkyn Hamley has risen from a wild country girl to become one of the finest students at the Academy of the Air. But Duke William, the new ruler of Oc, is obsessed with riding a winged horse himself, and his madness leads him to ignore his traditional duties to protect the Duchy. Now it's Larkyn who must take flight to save her country, her Academy, and her own life.

Driven by insane jealousy, Duke William is determined to found his own flying school, where the valuable flying horses of Oc will learn to bond with well-born young men-instead of arrogant women. Now, Larkyn Hamley and her beloved Black Seraph must gather all of their allies from the air to the ground. For if they do not soar now, none will ever see the skies again.