Mimics

The Mimic is an aggressive Hardmode enemy disguised as a Chest, appearing as the appropriate Chest type for the environment. Mimics will look exactly like Chests until approached, at which point they will chase the player.

Mimics take on the appearance of Chests that would normally be found in a given area: normal brown chests closer to the surface, gold chests found underground, and locked Shadow chests in The Underworld.

Mimics are immune to the Cursed Inferno and Confused debuffs.

Mimics can be spawned by the player using the Chest Statue, but they will not drop any coins or items this way.

Identifying

 * Using a Hunter Potion will make Mimics glow like any other enemy.
 * Using a Spelunker Potion will not make Mimics glow like real Chests.
 * Mimics disguised as Gold Chests or Shadow Chests will not sparkle as the real versions do.
 * If a new Chest appears at any point in an area you know well, it is probably a Mimic. Naturally-occurring Chests are only placed at World creation.
 * Mimics can sometimes be identified by the fact they do not always perfectly fit the tile 'grid', and thus do not line up perfectly with blocks, as normal Chests would.
 * Real Chests will never be situated directly over any empty tile spaces, so if you see a Chest on top of any empty tiles (including those resulting from sloped land), it is a Mimic.
 * Real Chests will never spawn on top of other background objects, such as Life Crystals, since real chests are background objects themselves.
 * Shooting a stationary disguised Mimic with a ranged attack will cause it to become active and start chasing the player. Suspicious-looking Chests can therefore be tested from a safe distance by firing at them.

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.