Forgotten book title. A girl gets the power to change into animals..Finds one friend he too can change.?

In the book a young girl is given an animal pelt on her birthday. (i think it just randomly appeared on her bed) She touches the pelt and then finds she can later turn into any animal she wants. Later in the book she turns into a chipmuck I think......and encounters a very playful squirrel. Some time later in the book she finds out that squirrel was another kid that could turn into animals as well. (the boy that can change into animals I think had a pact with the rats I'm not sure) When the Boy and girl meet as humans he tells her the power dosent last forever that it goes away at a certain age ( i forget what age) Then the boy and the girl are told to go to the artic to stop some kind of monster that had been causing trouble ( i foget who told them to go to the artic but the monster I think was a yeti) Well at this point in the book I skipped ahead and read that the boy at the end turned into a Pheonix.. So that is as much as I remeber if anyone can tell me the title of this book I'd be ver grateful.