This is what Wise Geek had to say about witch familiars.
In legend, a familiar or familiar spirit is a supernatural being that helps and supports a witch or magician. Traditionally, a familiar is an animal, but some are said to be humanoid. Familiars often have special powers of their own. When witchcraft is portrayed as a type of communication or alliance with evil forces in order to gain magical powers, a familiar may be considered a type of demon.
The stereotypical familiar in Western culture is the black cat. Because black cats are strongly associated with witchcraft, there are a number of superstitions regarding them. A black cat crossing ones path is said to be a portent of doom, for example. A familiar may be nearly any animal, however. Other common forms for a familiar to take are a dog, an owl, and a toad.
In the days of widespread persecution of witches, every witch was believed to have a familiar, and close animal companions were sometimes considered proof that a person was a witch. In addition to animals, humanoid creatures were believed to serve as familiars; sometimes, these familiars looked like regular people, and sometimes, they were said to be odd in appearance, having some deformity or resembling stereotypical images of demons. Legends of this period also often attribute the habit of drinking blood to familiars. Familiars were considered at least as dangerous as witches, as they were thought to be supernatural beings that looked like normal animals and could spy or wreak havoc for their witch without being easily detected.
While Christians traditionally interpret familiars as demons, to Wiccans and Neo-Pagans, they are more like the Christian concept of a guardian angel. A witch's familiar can be his or her closest companion, offering moral support, special knowledge, and/or physical healing. Wiccans may seek a familiar through meditation or divination, but most do not believe that familiars can be summoned, contrary to traditional Western legend regarding witches and black magic. Author Phillip Pullman offers an interesting take on the idea of the familiar in his His Dark Materials trilogy, in which every human has a "daemon" in animal form that has parallels to various cultural interpretations of the soul.
Now Emma Wilby who wrote Cunning Folk and Familiar Spirits: Shamanistic Visionary Traditions in Early Modern British Witchcraft and Magic.(Which I recommend to anyone interested in familiar information from Britain.)
In the British accounts from the Early Modern period at least, there were three main types of encounter narrative related to how a witch or cunning person first met their familiar. The first of these was that the spirit spontaneously appeared in front of the individual while they were going about their daily activities, either in their home or outdoors somewhere. Various examples for this are attested in the sources of the time, for instance, Joan Prentice from Essex, England, gave an account when she was interrogated for witchcraft in 1589 claiming that she was "alone in her chamber, and sitting upon a low stool preparing herself to bedward" when her familiar first appeared to her, while the Cornish cunning-woman Anne Jeffries related in 1645 that hers first appeared to her when she was "knitting in an arbour in our garden".
The second manner in which the familiar spirit commonly appeared to magical practitioners in Britain was that they would be given to a person by a pre-existing individual, who was sometimes a family member and at other times a more powerful spirit. For instance, the alleged witch Margaret Ley from Liverpool claimed, in 1667, that she had been given her familiar spirit by her mother when she died, while the Leicestershire cunning-woman Joan Willimot related, in 1618, that a mysterious figure whom she only referred to as her "master", "willed her to open her mouth and he would blow into her a fairy which should do her good. And that she open her mouth, and that presently after blowing, there came out of her mouth a spirit which stood upon the ground in the shape and form of a woman."
In a number of accounts, the cunning person or witch was experiencing difficulty prior to the appearance of the familiar, who offered to aid them. As historian Emma Wilby noted, "their problems… were primarily rooted in the struggle for physical survival - the lack of food or money, bereavement, sickness, loss of livelihood and so on", and the familiar offered them a way out of this by giving them magical powers.
In some cases, the magical practitioner then made an agreement or entered a pact with their familiar spirit. The length of time that the witch or cunning person worked with their familiar spirit varied between a few weeks through to a number of decades. In most cases, the magical practitioner would conjure their familiar spirit when they needed their assistance, although there are many different ways that they did this: the Essex witch Joan Cunny claimed, in 1589, that she had to kneel down within a circle and pray to Satan for her familiar to appear while the Wiltshire cunning woman Anne Bodenham described, in 1653, that she conjured her familiars by reading books. In some rarer cases there were accounts where the familiars would appear at times when they were unwanted and not called upon, for instance the Huntingdonshire witch Elizabeth Chandler noted, in 1646, that she could not control when her two familiars, named Beelzebub and Trullibub, appeared to her, and had prayed for a god to "deliver her therefrom".
(1)Wilby 2005, p. 60.
(2)Wilby 2005, pp. 60-61.
(3)Wilby 2005, pp. 66-67, 70-71.
(4)Wilby 2005, p. 77.
(5)Wilby 2005, pp. 77-78.
Now having grown up on Hillbilly Folk magic there was a few of the Granny’s that claimed familiars. One I remember very well, it was Granny Nellie from the Eleven Point Divide who was always accompanied by a huge dog where ever she went his name was Ralph. I never heard the dog bark or growl and it always seemed friendly enough when we would happen across each other path while in Thomasville or down by the river crossing or bridges. The odd thing was she had the dog as long as I could remember and it never seemed to grow older.
After I returned to Thomasville as a grown woman with half grown children to show them where I had been raised up. We stopped by the Thomasville store which reminded my oldest of the store from the T.V show the Walton’s to give you an idea of the size of the town of Thomasville.
When we went to leave I seen Granny Nellie coming down the road with her dog at her side and I stated to Rork the owner of the store that I see Miss Nellie still had pups from old Ralph and he looked a bit uncomfortable and said, “ I ain’t to sure if that is a pup of Ralph’s or Ralph himself.” I asked, “Why’s that?” He states, “Ain’t never seen her with no pup and that old lady has always creeps me out any how.”
A few years later we found ourselves in Thomasville again for a gathering and I asked after Granny Nellie and was told she was found dead about two years back along the road from her place up on the divide. I asked after her dog and was told the dog was never found and assumed to have run off. Seeing I was in mixed company I didn’t state my opinion of what had become of the dog. But has a strong feeling that his work as a familiar was done with her passing.