Mimic

Mimics were created as a lethal protection against looters. They are indistinguishable from normal chests until opened.

You can find them in dungeons or as encounter "treasure chest".

Detection
Using an Identify Scroll will let you know whether a chest is a mimic or not.

Combat
The fight against one mimic is a one enemy fight.

Mimics can steal from you.

Trivia
Mimics have been a Dungeon Master's favorite, first appearing in the Dungeons & Dragons tabletop role playing game back in 1974. Basically a creature hiding as a chest, waiting for the unsuspecting adventurer.