This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Familiar (English)
In European folklore of the medieval and early modern periods, familiars (strictly familiar spirits, as "familiar" also meant just "close friend" or companion, and may be seen in the scientific name for dog, were believed to be supernatural entities or spiritual guardians that would protect or assist witches and cunning folk in their practice of magic.
When they served witches, they were often thought to be malevolent, but when working for cunning folk they were often considered benevolent. The former were often categorized as demons, while the latter were more commonly thought of and described as fairies. The main purpose of familiars was to serve the witch, providing protection for them as they came into their new powers.