Beings as it is October I felt a little magic and spookiness was in order!
Farmiliar are animals, often cats, that act as magical guides, guardians, and fur friends for the magical person(Witch, Wizard, Warlock, NB magical folx).
The most well known example to me is Salem the cat from Sabrina the Teenaged witch, Sabrina the Animated Series, and Chilling Adventures of Sabrina. In the sitcom and animated series Salem is a convicted warlock forced into a cats body for penance. He acts as comic relief, guide, and trouble maker throughout the series. In the Netflix remake, Salem does not speak but he shows up as a guide at important points and tries to warn Sabrina about danger.
In Charmed the sisters have a cat named Kit that is a witch shifted into a cat who tries to protect and warn the sisters away from danger. Then for some reason she becomes a crazy cat lady.
In role playing games you can have a familiar. When I played Vampires, I had a fox familiar ghouled into my service.
I think what the role is missing in all of these is a more active role. In my head the familiar goes to lessons with their person, helps guide them, grows with them as a practitioner, while also protecting them from harm. The familiar is an indispensable friend to the practitioner as magic can be solitary.