Mimics are a class of 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, making them a common cause of death early in the game; small mimics rank #6 on the list of top deaths on NAO.
Large and giant mimics have a sticking attack, which prevents the player from escaping the confrontation. Walking into a concealed large or giant mimic, or finding it by searching when adjacent, will automatically stick you. The sticking attack can be protected against with magic cancellation.
Eating a mimic's corpse makes you imitate a pile of gold for a short time; if hallucinating, you will mimic an orange instead. This lasts for 20 turns in the case of a small mimic, 40 for a large one, and 50 for a giant mimic; in all three cases, the nutritional value is 10 times the time spent, so it's a net gain. Turning into gold is a weak form of polymorph: you won't polymorph if you have unchanging, and you'll fall off your steed if you are riding. However, armor and such is not affected and your stats are unchanged. This breaks polyselfless conduct as of 3.6.0 (although it did not in 3.4.3).
A mimic is uncloaked when wearing a ring of protection from shape changers.
There is a (Dungeon level)% chance that a square in a shop that would normally have an item will instead get a mimic imitating a shop item.
Two giant mimics, imitating boulders, always appear on the final level of Sokoban.
Of course, mimics can be randomly generated like most monsters can. Mimics can be items (in rooms), boulders, unaligned altars (in temples), and even closed doors (in doorways) or staircases (in rooms) in the dungeon. For example, the Rogue quest home level has four staircase locations, but three of these (chosen at random) are actually mimics.
Mimics sometimes display as "mimic or strange object" (],). In shops, the chance of this happening is 10*(10 - dungeon level)% on dungeon levels 1-9, and is 2/17 otherwise. You should not attempt to walk onto such objects, as they are always mimics. It is dangerous but not unusual to confuse these with the armor symbol, which is [ outside the Rogue level, or with a chest () in the tiles interface.
If you have warning, anything that is an item when you can see it and a warning when you can't may be a mimic. If you are warned of a monster, and then see an item where the warning was, it is probably a mimic (or a snake or insect hiding under the object). Do not walk onto the mimic to discover it, however, as doing so will cause you to stick to it. Throw a rock or piece of gold at it, instead.
Priests and others with a healing spell can cast it at an object; this will produce a message such as "The wand seems a more vivid white than before," if directed at a mimic.
A method of checking shops for the presence of mimics is to throw 1 gold piece across the shop, as it is not lost but added to your store credit—but be careful that you don't anger the shopkeeper by hitting him/her with the coin.
Another method is the "One of these things is not like the others" method; if you walk into a shop that sells specific items and notice something that doesn't quite fit, chances are it's a mimic. This goes especially for chest- or armor-like "strange objects".
Applying a stethoscope on a cloaked mimic will also reveal it and wake it.
While mimics can be dangerous, they are exceedingly slow and are worth a fairly large amount of experience. Characters with relatively high starting HP and AC, like valkyries, samurai and barbarians can attack more safely, but even the weaker classes can defeat mimics without taking a single hit if you are careful. Assuming you are unburdened, you should get at least 4 moves for the mimic's every 1 move. Just stay a short distance from it until it comes towards you, then attack three times, retreat one step, and repeat. Be careful of bumping into another mimic! Also, it is recommended that you wake up the mimic from a range (throw a gold piece or rock at it) to ensure that you don't get hit by it. Remember that mimics are worth enough experience points to make you advance a level; therefore, to take best profit of them it is usually best to wait till you are level 3 or more before dispatching them.
If you find one outside a shop, shoot it repeatedly. This works best for rogues and rangers, though wizards can do this well, too. However, beware of using ranged attacks if you are in a shop—any missiles will automatically become the store's property, if the store buys that type of item. Spells of force bolt can destroy fragile items like potions—which you will have to pay for—and of course will hit the shopkeeper if he/she is standing behind the mimic.
Mimics are a surprising candidate for early genocide or extinction. Since mimics are sometimes generated in place of shop items, preventing the game from generating more will make future shops better-stocked. This is a relatively small effect except in Orcus-town, where it will greatly increase the number of real items in the abandoned shops.
Being polymorphed into a mimic (for instance, by a polymorph trap), is a rather bad situation. Mimics are not fast, do not have a lot of strength, and the large and giant versions even make you tear out of your armor and/or cloak. Furthermore, mimics do not have limbs, and are therefore unable to wield a weapon. The only thing which may help is attempt to hide via the #monster command. This will make you mimic an object, and is not particularly useful, since monsters are able to easily find you. Returning to your original form is the best option.
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.
He walked for some time through a long narrow corridor
without finding any one and was just going to call out,
when suddenly in a dark corner between an old cupboard
and the door he caught sight of a strange object which
seemed to be alive.