Mimic

s are a type of enemy that look like wooden, frozen, golden or locked shadow chests. They will stay still initially, but will reveal themselves by jumping and biting the player if he/she moves within their attack range. Another way to reveal a Mimic is to shoot it from a distance. If the chest is a Mimic, it will take damage and move to attack the player, revealing itself in doing so.

Unlike regular chests, Mimics will not glow while under the effects of a Spelunker Potion, making it easy to differentiate between a Mimic and a standard chest. They do, however, glow under the effects of the Hunter Potion. Similar to their passive counterparts, Mimics are usually found underground, but can sometimes rarely appear on the surface.

The Mimic is a Hard Mode monster, so it cannot be encountered naturally before the Wall of Flesh is defeated. Mimics can be spawned by powering the Chest Statue, but they will not drop any loot when created this way.

Ice Mimic
Since update 1.2 Ice Mimics can be found in the Hardmode underground snow biome. Known loot for now goes as follows:
 * Flower of Frost
 * Ice Bow
 * Ice Sickle
 * Frostbrand

Trivia

 * Monsters that trick the players by disguising themselves as treasure chests are common place in many RPGs, mainly dungeon crawlers where treasure chests are common enough to make the player unsuspecting. The vast majority of them are also called Mimics.

Update Info
v1.2

Ice Mimic now generates in underground snow biome and has different loot pool than regular mimics.