Nancy Springer 
Wings of Flame 

Swords and sorcery fill this fantasy about an outcast girl who teams up with a prince to break a supernatural curse that could destroy their warring kingdoms  In the kingdom of Vashti, a shuntali is an outcast—an unperson. One day, a young warrior gallops into the city astride a magnificent blue stallion. He is Prince Kyrem of Deva, and he has come to Vashti on a mission of peace between the warring kingdoms. En route, he is attacked by brigands only to be rescued by a rag-tag stable boy who seems to have a way with horses. Kyrem names him Seda, unaware that the shuntali is really an orphaned girl who has disguised herself in order to survive. Now Seda, Kyrem—whose people possess mystical powers—and the peace-loving healer and magician King Auron of Vashti must join forces to save their nations from the Nameless One. But deep within a cave atop a steep mountain, the last of a vanished race gathers his demonic forces to unleash his revenge on the kingdoms bound together by the great horse-god Suth.   A lonely girl with no past or future and a prince who wants to use his powers for good must find the love and magic to battle hatred and create a peace that has been a long time coming.