Cracked has an illustrated list of 15 Retarded Dungeons and Dragons Monsters. The post is from 5 years ago but is still fairly entertaining. I didn't recognize the vast majority, so I assume they are either 3e or 4e creatures (or possibly from the later 2e monster books).
That's number 14, the Gelatinous Cube at the top of the page.
It got me thinking about the lamest AD&D creatures. I think the Flumph get's high points here, but I still love the fuckers. Would never use them in game, but I enjoyed their appearances in OoTS.
So, what are your favorite retarded or lame AD&D creatures (I'm going to work on the assumption that most if not all will be coming from the Fiend Folio ;)