Yrael is an entity in Garth Nix's Old Kingdom Trilogy, first appearing in Sabriel. His true identity as the cosmological entity Yrael is not revealed until the final pages of the final book of the trilogy.
When first presented, Yrael is in the form of a small cat by the name of "Mogget", who is obviously of a magical nature, since he can speak. The heroine of the first book, the eponymous Sabriel, finds him inside of her father's house, and recognizes him as a magical creature. He is also magically bound to serve her family line, the Abhorsens, necromancers charged with putting the dead to rest. Mogget serves the family with some resentment, and can become a dangerous free magic creature if his collar is removed. His resentment is never explained, although it goes along with his feline form, since he acts like a typical cat, being very lazy and insolent, along with a cat's penchant for knowing more than it should.
The climax of the third book details the somewhat gnostic cosmological backstory of the books. The world was formed when nine "bright shiners", spirits somewhat like archons, were in a war with each other. The most powerful spirit, Orranis, was a spirit of destruction, and seven of the other spirits joined together to defeat and bind Orranis. One of the spirits, Yrael, was neutral in the conflict and was therefore punished by being bound into the form of Mogget. In the cataclysmic battle of the final book, as Orranis tries to defeat his binding and regain his full power, Yrael decides to join against him, despite his resentment at being magically bound. He gives his reasoning for wanting to protect the world against destruction as
"Life...Fish and fowl, warm sun and shady trees, the field mice in the wheat, under the cool light of the moon. All the---"
Nix also writes that Yrael is "more Mogget than it ever knew", suggesting that its time incarnated as a cat, with a cat's simple appreciation of food and rest gave the amoral spirit a sense of value it didn't even realize.
It is just such oddities, plot twists and surprises as this, having a major cosmological spirit in the form of a bratty cat, that make Nix's books so rewarding and make him a unique fantasy writer.