Wings of Flame — Nancy Springer

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.