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 are probably a reference to mimics in other popular games, like Dark Souls or Terraria.