Fun:Gelatinous cube

One of Dungeons and Dragons' eternal mysteries: how in the world could a gigantic cube of caustic gel evolve in any environment other than a man-made dungeon?

Rebuttal There's always a 10' x 10' room for Jell-O

Better rebuttals Cubes are just industrial cleaners for dissolving those messy TPKs. Or causing them. Or, most likely: Insane wizards, who spend all their time gazing upon things NOT MEANT TO BE, enjoy jello as much as the next demented magic-user.

 Even Better Much worse rebuttal These things didn't evolve, they were intelligently designed!

Yet Another Rebuttal It's the hat and the broom scenario all over again. Wizard creates a creature that can move and clean on its own to help tidy the dungeon (A clean dungeon is a fun dungeon!) before the PCs step inside. Gives creature the ability to asexually divide, therefore covering more area.