Biker dwarfs in a dungeon

In my head, these dwarfy folks have just hit the first level, full of enthusiasm, energy, and greed for gold, only to be confronted by… something. What the hell is it? You tell me! One of the fun aspects of dungeons, at least for me, is to play around with expectations. I suppose this monster could be a pudding or a slime, but I think there's a great deal of suspense and excitement when a Referee tosses out the ol' standard bestiary or monster manual and uses brand new critters instead. That keeps players on their toes, never quite sure what they're facing.