A familiar is a Lux that a witch has summoned into the body of an animal.
Lux, being masters of mimicry, can adapt into any animal form, but their behavior might not always match their acquired body. A Lux in a comfortable vessel becomes trainable and intelligent- typically comparable to a dog- so even a hamster could become an assistant. Since Lux experience time across alternate dimensions, a familiar's paw on a crystal ball is invaluable. Corruption of a demon while restricted to a mortal body is also impossible, so there is little worry for a familiar to become a Mud Beast.