I ran an old adventure from Dungeon that featured a nilbog. That's the little goblin from the old Fiend Folio that makes everyone around it act in a manner contrary to their intentions, and can only be killed by healing magic.
The creature was hiding in a village, and the whole town was going nuts because of its aura of contrariness. That adventure was a blast.

I still remember the PCs walking into a shop and having a session of reversed haggling with the storekeeper, or the party fighter (at the beginning of combat) failing his Will save, dropping his swords, and charging their opponents for a hug.