Mimic

Mimics are monsters whose main defense is to imitate an item or dungeon feature. Mimics can be small, large, or giant, and all three kinds are often located in shops and Sokoban. They are slow but powerful hitters and often kill low-level characters who encounter them.

Eating a mimic's corpse makes you "imitate" a certain item for a few turns.

A mimic is uncloaked when wearing a ring of protection from shape changers.

Mimics are a surprising candidate for early genocide. One reason to genocide mimics is to increase the chance of getting useful items in Orcus Town.

Strategy
If your experience level is low enough, then mimics will display as "mimic or strange object". You should not attempt to pick up such objects, as they are always mimics. It is dangerous but not unusual to confuse these with the armor symbol, which is.

If you find yourself fighting a mimic, then hit and run is generally the best strategy (as long as the mimic is not stuck to you). If it is, try Elbereth.

While mimics can be dangerous, they are exceedingly slow and are worth a fairly large amount of experience. Strong characters, like valkyries, samurai and barbarians can use mimics found in early shops to gain levels quickly. If you find one outside a shop, shoot it repeatedly. This works best for rogues and rangers, though wizards can do this well, too.

Encyclopedia entry
The ancestors of the modern day chameleon, these creatures can assume the form of anything in their surroundings. They may assume the shape of objects or dungeon features. Unlike the chameleon though, which assumes the shape of another creature and goes in hunt of food, the mimic waits patiently for its meals to come in search of it.