In Harry Potter, a familiar is a magical animal that acts as a companion and helps a witch perform spells. A familiar can take the form of an animal, a plant or even a human. It is believed that the medieval belief that Satan granted demon servants (in animal form) to witches influenced the creation of the familiar concept in Harry Potter. Familiars are most often seen in the forms of animals. They can take on the shape of any creature but most commonly a dog, cat, various rodents and weasels, toads and small birds.
Harry's snowy owl Hedwig is one of the most well-known examples of a familiar. Hedwig demonstrates incredible loyalty throughout the series and is a useful companion for Harry on many adventures. She can deliver mail without knowing an address and intuitively knows when her owner has moved. Hedwig also carries important information, such as Tom Riddle's diary, to Harry.
Another familiar is Buckbeak, a winged hippogriff, which is half-bird and half-horse in the Harry Potter universe. This creature is very strong and can fly. It is a vital member of the wizarding community when the basilisk, a legendary reptile from European mythology that can reach 50 feet tall and deliver deadly venom, attacks Hogwarts.
Ron Weasley's pet rat Scabbers is another familiar. Hedwig and Buckbeak are both based on birds but have more human qualities, such as personalities and abilities. Ron's rat, for example, is a bit of a lazy lout who loves to nap and has a ragged left ear. Scabbers is actually the animagus form of Peter Pettigrew, who was in hiding from the dark lord.