Leprechaun

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)