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. The scroll will give off a white light with sparkles if it is a chest, and a red light with skulls if it is a mimic.
Combat
The fight against one mimic is a one enemy fight. The mimic will always go first and will attack the party member used to open the chest.
Mimics will occasionally use an ability called Chomp. If successful, this attack will destroy a random item belonging to the targeted character. Even if it is defeated, the item cannot be reclaimed (as is the case with Cutpurses, for example).
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.
When a Mimic is killed it drops loot that you can find in an normal, non-carnivorous chest.