While the concept of “the familiar” is just that, it owes more to the delusions of Christian theologians or popular literature than having in any basis in fact. For those of you unfamiliar with such things, these were an animal, supposedly given by Satan to a Witch as a watchdog, messenger or assistant. While trial records are full of them, none of the spells which survive from this time mention them. What may have distinguished a magical practitioner from the general public was a certain empathy with the not human spectrum of life, a Dr. Doolittle effect, as it were.
The same holds true today. While your dog or cat might be the one member of the family that really understands you, it’s rare for them to take any real interest in the practice of magic.