Mimics are mischievous entities which harmlessly toy with passing adventurers. They disguise themselves as randomly generated (often high quality) loot or as dungeon features. However, once you come within a one-tile radius of them, they will "vanish and cackle," disappearing from the game forever. Item mimics can be disappointing, but not dangerous. Dungeon feature mimics, however, can be hazardous in rare cases: if you find yourself fleeing from an opponent and in need of a staircase to a safe floor, be aware that an untested staircase may turn out to be illusory.
To clarify, mimics are not monsters, and cannot be killed in any way.