The Dark Horse by Marcus Sedgwick

A girl snatched from the wolves, a sealed box that can only be opened by one person, a sinister stranger, and a boy destined to lead his clan are woven together in this riveting story of betrayal and ancient magic.
Set in a distant time, in a distant place, the Storn live quietly. But on the day of the wolf hunt their lives will change forever. That's the day Sig rescues a small, ragged, howling child, more like the wolves she came from than a human being. They call her Mouse, but who is she really? What is her secret? Years later they find out. The shocking discovery of her true identity brings to life a terrifying legend and plunges them all into treachery and a cruelty from which there is no escape.