The river's gift

Taken from Mercedes Lackey's website:

In The Rivers Gift, a young woman uses her healing gifts to help a magical horse-like creature--and receives, in return, the greatest gift of all...

Fifteen-year-old Ariella, lady-to-be of medieval Swan Manor, possesses magical healing abilities that she practices on the animals in the forest adjacent to her father's lands. One day a magnificent black horse emerges from the nearby river in need of her ministry. The horse is Merod, and he is a more-than-mortal Kelpie, a magical being who converses telepathically with Ariella. Distrustful at first, he warms to Ariella gradually, which is vital to her when, after her father's sudden death, she is taken away by a brutish cousin to be his bride.