Fiendish gnome with (su) Grant Luck: A leprechaun has the supernatural ability to grant either a good luck or bad luck giving the target either +2 or -2 to all attack rolls, saving throws, and skill based checks.
Oh Lenny the leprechaun, how the wild elf PC in my campaign hated his tricksy ways so. In 1e they only had something like a d4 hp but they had polymorph any object at will. And I was a DM willing to run with that.
Bedlam the Leprechaun made a few appearances in my 2nd Edition AD&D campaign. Turned a bunch of orcs into teletubies, if I recall correctly. Oh Middle School Gaming, how I laugh at thee...