While the witch’s familiar can be found in other countries (I found some in some French cases), it represents one of the features that most separates the English witch from her continental counterpart.
The era of the witch trials in England came before the creation of the idea of ‘Britain’, from the traumas of the Reformation in 1538, gradually losing judicial support from the mid-17th century onwards.
A part of the reason for familiars is an extension of the idea birthed in Continental witchcraft that the witch herself was powerless: witches did not truly wield magical power, but instead they made bargains with Satan, who would send demons to invisibly watch over them and reproduce magical effects when they performed certain ritual actions.
In the classic Continental trial this is expressed in the image of the Sabbat, where witches meet and worship Satan in a series of increasingly degrading and humiliating rituals, culminating with the ‘foul kiss’ where witches would kiss Satan on or under the anus.
English witch trials feature the Sabbat less often. Scottish trials tend to be far more traditionally continental in character, with the North Berwick Witch Trials hinging on a lurid Sabbat at the North Berwick Kirk that could have come directly from the pen of Conrad of Marburg. By contrast, without searching my notes, I can only think of a single trial with a strong Sabbatic image: the Windsor trial of Elizabeth Style, where the witches involved confessed to meetings where they would agree their nefarious activities.
With the Sabbat being a less common image, the familiar seems to take a similar disempowering role. It also gave the searchers the hope of finding physical evidence. Even if an animal designated as a familiar could not be found, which they usually could not, the ‘teat’ the animal suckled from could usually be located on the body. Since the teat was a magical thing, not a natural part of the anatomy, it could be found anywhere… although it is of interest that the most common locations and description of witches’ marks conform to those of sebaceous cysts.
A final, especially sad feature of the English witch trials was the use of children as witnesses. While the influence of King James Stuart led to the discrediting of child witnesses in the early to mid 17th century, we see a significant use of child witnesses in the earlier part of the witch trials. In the 1582 trial of the St. Osyth witches Ursula Kemp’s eight year old son Thomas Rabbet gives us the names of four familiars whose names have clearly come from the mind of a child: Tiffy, Jack Pigin and Tyffin. We see similar image from the interrogation of James Device during the witch trials at Pendle, with a second era of the familiar during the brief activities of Matthew Hopkins and John Stearne in the South East of England in the 1640s.