Note: Do not rely on this information. It is very old.
FamiliarFamiliar, Familiar Spirit, the supernatural attendant and servant of a magician, or witch, generally assuming the form of one of the lower animals. This meaning seems to have arisen from the particular sense of the Latin famulus = a servant of the gods (Ovid: Metam., viii. 272). The "familiar spirit" of the Authorised Version appears to have been an indwelling and controlling influence rather than an entity external to the witch or diviner; and this difference will be clearly brought out if one compares the passages in which this expression occurs with the corresponding ones in the Vulgate. In the Middle Ages the notion of familiar spirits was widely diffused, and during the witch mania any old woman who kept a pet cat or dog was almost sure to be denounced as a witch. In the Faust folk-legend and in Marlowe's version Mephistopheles is a true familiar spirit; in Goethe's poem he is The Devil, der Junker Satan, as he is called by the old witch who brewed the broth that made Faust young again.
Click for more about Vulgate