A familiar animal, in folklore, is an animal believed to be possessed of magic powers such as the ability to change its shape. It may be a temporary form assumed by a spirit, devil or trickstergod.
In tribalshamanism and also in spiritualism it is believed that astral travelling is done in a special "body" or "form" which is able to travel out of the physical body and go around the world, spying out the land. The astral form is often believed to be that of an animal. This sort of belief is very ancient and is recorded in Egyptian Hieroglyphics.
Sometimes, familiars are imps, zombies or demons, or some sort of spirit that takes the form of an animal, but usually it is an animal that has bonded with an individual, that has been karmically attracted to that person and volunteered to work with them as a familiar.
Familiars usually stay with their companion until death, or they are unsummoned, and then they disappear.
This relationship is not as strong as that of a familiar, and certain things cannot be done because there is not the psychic/magical link between the animal/beast and the person, but there is still helpful tasks and an "understanding" that takes place between the companion and animal.