|[Insert obligatory joke about how these are clearly the same annoying creatures which cause you to dance irresistibly, like my friend Otto]|
A room full of Pixies is my choice for today, to round out this week of favourite monsters. Pixies can do next to no damage (in 3.5 its 1d4-2), but my GOODNESS they can screw a party over so hard. Greater invisibility plus damage reduction 10/cold iron makes them nearly impossible to kill, even with only 1d6 hp. Entangle as a spell and arrows of Sleep and Memory Loss make for a vicious combination. They will almost never kill a party, but after half an hour of confusion and chaos, any player will be reduced nearly to tears.
Nasty creatures. Which is why they’re so much fun to play! Also, letting the party get tangled up and messed around by a group of pixies for a little while is a great prelude to unleashing an Owlbear on them. Muahahahahaa.