Witch's familiars are former witches who have committed serious crimes and been turned into animals as punishment by The Witches Council. The sentence usually lasts a century or more, depending on the severity of the crime committed.
Cats are the most common form of witch familiar, however various other animal forms have been used (Dogs, camels, guinea pigs etc.).
While in this form, a witch familiar has no access to any of the magic they had as a witch, but regain their magic once the sentence is served and human form is regained. Black witch cats are known to have the power to scare the living daylights out of people on Friday the thirteenth.
Witch Familiars are made to live with a witch for the duration of their sentence. Occasionally the witch is selected due to their connection to the crime (ex. Hilda Spellman was made to house Salem Saberhagen because she was the head of the refreshments committee in his regime).