Roverandom by J.R.R. Tolkien

Rover (a.k.a. Roverandom) is a small black and white dog who lives a happy life with his owner. One day, after upsetting the powerful wizard Artaxerxes, Rover is turned into a toy and forced into a toy store. He then must travel past the moon, ocean, and greedy little children's pockets in order to return not only to his home, but also to his original size.