I'd like to request a bit of an unknown monster, the Bog Imp from Heroes of Horror (p.142).
They're tricky little evil fey who can burrow as fast as a horse can run, taint nonmagical water, cause sinkholes to drag people underground, and corrupt elves into yet more bog imps. What's more, they're somewhat unique in that each group abides by different codes of behavior; some might allow travelers to pass unmolested if they can guess the imp's name, while others might have stranger laws.