I know you said it didn't count, but I'd have to say Dragons , I mean, the game is called Dungeons and Dragons! And I've never seen a party get anywhere as excited about killing a beholder or gelatenous cube, so those would be rather lame choices as the Quintessential monster. If dragons are out then I would next pick Drow Elves (although the concept of evil dark elves actually existed long before the game).