Mimics, or ???, are enemies in Final Fantasy X, and are found in the Omega Ruins. They are named so for their behavior of mimicking Treasure Chests, and the shapes of fiends they take on. There are four different shapes of the Mimic, each one having a different set of strengths and weaknesses. When encountering them in a battle (always with a Zaurus), they will show up in the form of a treasure chest. Steal from it (not Mug, as it'll destroy the chest), and if there's Nothing to steal, it'll "change" into the Mimic, a fiendish look-alike from a pile of bones, rocks, and other debris.
It is one of the few enemies in the game that can not be "captured" for the Monster Arena, as well as being unable to escape from the battle once they have changed into their fiend-like shapes.
edit Shapes and Strategy
While not one of the hardest enemies in the game, it is good to learn all of the weaknesses to take them down quickly and easily. Especially useful to know since they are one of the best sources of gil in the game, with each one dropping 50,000 gil, double that if a party member has the weapon ability Gillionaire. So, knowing what kills each shape quickly will guarantee survival along with quick gil.
Mimic (1): Takes on the shape of Ruminant fiends. Has the highest Strength and HP out of the four shapes. Both physical and magic attacks will work, as both defensive stats are low.
Mimic (2): This one is in the shape of Mech Machina. Has the lowest Strength out of all the shapes, but good Defense, Magic Defense, and the highest Speed. Using either physical or magic attacks against it will work.
Mimic (3): This Mimic is in the shape of the large Roc fiends. Has the highest Defense out of all the shapes but low Magic Defense, so magic is best used on it.
Mimic (4): In the shape of the Basilisk fiends. Highest Magic Defense, but low Defense, so beat it down with weapons.