Leprechaun
Facts
A leprechaun is a NetHack monster whose main "attack" is to steal some of your gold and teleport away. This makes them, primarily, annoyances. Eating a leprechaun's corpse has a chance of conveying teleportitis. Leprechauns are usually sleeping when they are generated.
Leprechaun halls exist in the NetHack dungeon; these are special rooms filled with many, many leprechauns as well as gold.
Commentary
If you are a Knight, awaken leprechauns before attacking them to avoid alignment penalties.
Strategy
Leprechauns are best attacked with ranged weapons or spells. You can also prevent them from teleporting by hiding all your gold in a container or on a distant floor space. However, don't stand right on top of gold while fighting them, as they will grab it from under your feet and teleport away.