This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Fafnir (Germanic mythology)
In Germanic heroic legend and folklore, Fafnir is a worm or dragon slain by a member of the Völsung family, typically Siguro. In Nordic mythology, he is the son of Hreiomarr, and brother of Regin and Otr and is attested throughout the Völsung Cycle, where, Fáfnir slays his father out of greed, taking the ring and hoard of the dwarf Andvari and becoming a worm or dragon.
Before taking the form of a dragon, Fafnir's appearance is not explicitly described. It has been noted though that his brother, Regin, is described as being a dwarf in Norna-Gests þattr and resembling a dwarf, however, it is neither clear if relatives of dwarfs are also dwarfs, nor how dwarfs were conceived of as appearing. It has been noted that dwarf appearances were presented in the medieval period as highly diverse and they could even change their form.