Ettin

Ettins are a species of mutant hill giants from Dungeons & Dragons, whose only real claim to fame is that they have two heads. Each head has its own brain and thusly its own personality. The two usually can't stand each other and so ettins tend to spend a lot of time bickering and yammering to themselves.

But, since two heads are better than one, they've got a couple halfway decent powers out of the deal, like ease of two-weapon fighting, enhanced ability to spot people sneaking up on them, and making sure that one head's always awake and keeping an eye out for pesky adventurers. Some editions imply they're actually the result of Demogorgon's influence, since, you know, two heads.