Rockjaw, Master of the Eastern Border (Bone, Book 5)

In their most popular adventure since "The Great Cow Race", Fone Bone and Smiley strike out into the wilderness to return a lost rat creature cub to the mountains. They meet new allies - a scrappy young band of animal orphans (their parents were eaten by the stupid rat creatures) and a mysterious new foe: the formidable Rock Jaw, a giant mountain lion who fancies himself the master of the eastern border. Together they uncover the riddles of nature and before it is over, two great titans of the realm meet in an earth-shattering clash that will determine the fate of the land.