A leprechaun wizard is a stronger, spellcasting leprechaun introduced in SLASH'EM. They are always generated sleeping, and never in leprechaun halls. Note that leprechauns can grow up to be leprechaun wizards.
Leprechauns are very similar to brownies. They are mid-level spellcasters who are derived from a thieving monster, and more importantly, are always generated asleep. They are truly best left this way; although they cannot steal your items as a brownie can, waking them up means you will be facing a fast, moderately tough spellcaster who is more than happy to curse your items and teleport away. If you do find yourself facing an awake leprechaun wizard, it is advisable to ditch or stash your gold, to prevent it from teleporting away after every attack.