Mimics

The Mimic is an aggressive mob disguised as any kind of Chest. Mimics will chase the player when approached. They spawn after the defeat of Wall of Flesh or when a Chest Statue is activated. When spawned via the statue, no items are dropped. Mimics can take on the appearance of chests you would normally find in an area, such as normal brown chests closer to the surface, gold chests found underground, and even locked Shadow chests in The Underworld.

If a Mimic is hit by a ranged attack it will become active and start chasing the player. Mimics can sometimes be identified by the fact they do not always perfectly fit the 'grid' used, and thus do not line up with blocks as a normal chest would.

Mimics are immune to Cursed Inferno and Confused debuffs.

Trivia

 * Mimics first appeared in Dungeons & Dragons in 1977, and have appeared in countless RPGs since.
 * It seems that normal brown Mimics' only spawn requirement is a naturally generated dirt backwall; if you dig out a mountain but leave the backwall, Mimics can occasionally spawn.