Glaux wrote a wonderful article a couple of days ago on the American Folkloric Witchcraft blog about familiars, both corporeal and incorporeal. It's a topic we've been discussing for some time, and I am very pleased with the article and the information it presents.
Like many Witches, she and I both have corporeal familiars -- animals who are our pets who show a special interest in magic and energy work. Mine is George, one of my rabbits. He's the snarky one -- the Thelemic, free-will rabbit.
Rabbits actually have a long history as Witches' familiars, and Witches often chose the shape of a rabbit for shape-shifting. Both points born out in Judika Illes' wonderful compendium The Element Encyclopedia of Witchcraft. In it, she says that the animal most associated with European Witchcraft was the rabbit, not the cat. They are associated with the moon in many cultures, with fertility, and with a certain invisibility.
In fact, "pussy" probably referred to rabbit before it ever did to cat -- lepus being the Latin name for rabbit.
There is a record of the names of familiars given during the witch trials in Europe, and "George" is one of the names given. My George was named after George Milton in Steinbeck's classic tale of friendship and tragedy, Of Mice and Men. ("Tell me again about the rabbits, George.") He's a very literary bunny. He eats all the best books.
I also have a familiar spirit (an incorporeal familiar). It is the spirit of an animal I had known in life as a cat -- a black cat who looks like a cross between Glaux's Mr. Jinx and Miss Hex. She was killed just as I began walking the Crooked Path, but members of my household have seen her -- a spectral black cat in parts of the house where Jinx and Hex aren't permitted. I have also seen her as a young woman, just once. Only I have seen her in that form.