Since time began animals have been revered and worshipped as spirits of nature, known to the ancients as power animals or the animal guides of the Gods.
Many animals therefore became associated with various deities, i.e. Diana and the Hound, Hecate and the Toad, Proserpina and the Raven, Pan with the Goat, Athena with the Owl, the Spiral Goddess with the snail.
The ancients believed animals were closer to nature than humans, and would perform rituals and make offerings to their spirits in attempts to communicate with them.
Old shamans believed that all things and beings, particularly animals, were possessed of a spirit or soul, and that one could attract parts of their soul (thus their spirit and powers) with mimicry. To achieve this, they dressed in appropriate animal furs and feathers or wore horns and fierce looking masks while performing dance and imitating their antics.
The novice shaman would acquire his animal spirits on completion of his initiation. These he would send out on errands or to do battle on his behalf. However, if they failed or died then so too did the shaman. The shaman would keep and use the same animal spirits until his death, upon which time they would disappear or be passed on to aid his apprentice.
Given the animal kingdoms intimate relationship with nature, it's not surprising that 'witches' as they evolved should adopt certain animals as their own link to nature, spirits and deities. It wasn't until the Middle Ages and the rise of Christianity, however, that the witches pets and animals became thought of as agents of evil. As the persecution of witches began, so the church started teaching the concept that the witches' familiar was an associate of the Christian devil. They became demons and evil spirits in animal form, sent out by the witch to do their nasty bidding. It was also believed witches possessed the power to transform themselves into animals, in which guise they committed any number of diabolical deeds. Later they were believed to use animal products in spells, making potions and concoctions to aid transformation, gain power over nature, or even to harm and kill.
The most common animals associated with witchcraft were the: Frog, Owl, Serpent, Pig, Raven, Stag, Goat, Wolf, Dog, Horse, Bat, Mouse and of course the Cat, though virtually any animal, reptile or insect would be suspect. Obsession with the witches familiar was most prevalent in England and Scotland and was mentioned in numerous trial records of the period, particularly those related to "Matthew Hopkins", the infamous Witch Finder General. According to the ancient Witchcraft Act of 1604, it was a crime to "consult, covenant with, entertain, employ, feed or reward any evil or wicked spirit to or for any intent or purpose" (an act that Hopkins used with zeal when extracting confessions). He also used the so-called Inquisitor's Handbook which although offered no instruction concerning familiars in the interrogation and trial of witches, did acknowledge that an animal familiar "always works with the witch in everything". As such, it advised the inquisitor never to leave a witch prisoner alone, "or the devil will cause him or her to kill themselves, accomplished through a familiar".
With this in mind, Hopkins would tie the 'witch' up in her cell alone, watching secretly for the arrival of her familiar. If so much as a fly or beetle approached the poor woman it was deemed proof enough that she was, indeed, a witch!