The Lepra-Duck is a supernatural creature resembling an anthropomorphic duck and a leprechaun, with his name perhaps implying that he is both.
Little is known about the origins or powers of this supernatural creature, but the Lepra-Duck is certainly a powerful, magical being. He is capable of traveling forward and backwards in time, presumably at this will, and also seems able to teleport. He physically resembles both an anthropomorphic duck and a leprechaun. His name would seem to imply that he might be a hybrid between the two species. He is much shorter than other anthropomorphic ducks and also possesses the seemingly rare trait of green feathers, perhaps pointing to a leprechaun ancestry.
He is the owner of a wishing stone which is, according to one source and Lepra-Duck himself, the origin of Gladstone Gander's luck. In 1966, the Lepra-Duck grew tired of Gladstone's boastfulness and took the stone away from him, instead giving it to his cousin, Donald Duck. Donald proved to be as arrogant as his cousin, and the Lepra-Duck took his stone back. It is unknown what happened to him afterwards, but Gladstone has been shown to be lucky even after 1966. One might assume that Lepra-Duck chose to forgive his boastfulness and give the stone back to him or that Gladstone has another source of luck than Lepra-Duck's enchanted rock.
Behind the scenes
The Lepra-Duck first appeared and was created for The Battle at Hadrian's Wall, which was first released in May of 1966. Though this was his first and, presumably, only appearance, Gladstone Gander's luck, which he claims to be the source of, is a much older concept, dating to Race to the South Seas in 1949.