Leprechaun

Leprechauns were mythical creatues, resembling short men with beards, known for pulling pranks.

Kilvenny Odoyle was a human who believed himself to be a leprechaun while under the influence of Cephlie nanobots, which manipulated the reality of Avalon. He taught the First Doctor how to use the "magic" of Avalon. (PROSE: The Sorcerer's Apprentice)

Leprechauns were one of the mythical forms of the Sidhe. According to the Eighth Doctor, they were among the types of fae who were reputed to steal human children and leave changelings behind. (PROSE: Autumn Mist)

G. Syme used Leprechauns as an example of an imaginary folkloric trickster when trying to convince Honoré Lechasseur that the Doctor was similarly fictional, similar to "boojums" and "hobgoblins". (PROSE: The Cabinet of Light)