Leprechaun

The leprechaun is a conman who appears in the scrapped episode "The Knight and the Werewolf". Every year, he uses illusions to stage a fake battle between a paladin and a werewolf, taking bets on who will win from a crowd of spectators. The match invariably comes to a tie, and the leprechaun keeps all of the money.

His plot was foiled after Finn tried to attack the paladin and the werewolf to prove his own heroism, which reveals that they are both illusions. The leprechaun was tied up and taken to jail.

Trivia

 * The character is based on the leprechauns of Irish folklore, who are often depicted as being mischievous and greedy.