Green Rider by Kristen Britain
This first book of the Green Rider series begins with Karigan G'ladahon running away from the school she has just been suspended at when she stumbles across a dying messenger. As he dies he makes Karigan promise to deliver the message he is carrying. She accepts and finds herself thrown headlong in a series of dangerous adventures with people out to kill her and dependent upon the messenger's horse, who seems to know more than any horse should.
An entertaining start to a pretty good series, this book was a good example of fantasy fiction.