[manikus]

There are several AD&D creatures like this. The most famous is the 'mimic' which in the old monster manual had a picture of a treasure chest that had just grown a mouth and claws. Also, there was the 'cloaker' which pretended to be part of the dungeon ceiling and would drop on unsuspecting adventurers. There were also creatures that looked like stalagmites and would drop on adventurers as well.

-manikus