That would have to be the cat Oliver from A Well-Timed Enchantment by Vivian Vande Velde. When things start to happen in the book, Oliver is turned from a cat to a human (the most cat-like human ever). After the climax of the story occurs and things are set back to normal, Oliver is turned back into a cat, but the main character wants him to be human again. So she makes a wish. Needless to say, best cat ever!