Mimics are monsters who have the ability to have the appearance of any chest. They can be recognized in their chest form with by, while opening the chest, a short screech and slight jumping movements. While being looted, they take longer to open. Mimics can't be seen by Outlanders' Keen eyes ability.
Mimics will change appearance once fully "searched." Loot will be granted upon killing them.
There is a Stonewood quest called 'Treacherous Treasure' that involves finding and killing a mimic.