I've only recently become aware of some of these little-known 'hidden' and rare mobs and things that drop special vanity items. No I'm not talking about the [Glorious] rares, these aren't a part of any achievement and typically aren't 'rare', just hard to find.
So far I know of these four, but please tell me if there's any I'm missing:
Scotty's Lucky Coin, which drops from Scotty - Transforms you into a Thorn Sprite for 10mins.
B.F.F Necklace, drops from Huggalon the Heart Watcher - Puts a "best friends forever" buff on the friendly target.
Martar's Magnifying Glass, drops from Martar the Not-So-Smart - Summons a Gnoll guardian to fight and frolic with you.
Gokk'lok's Shell, comes from killing Gokk'lok - Makes you dance naked in a clam shell.