The Mimic is a fiend that is found in Omega Ruins. It is normally disguised as a treasure chest, always accompanied by a Zaurus. Should you attempt to steal from the chest, like normal, and the chest contains nothing, then it will transform into one of the Mimics (it’s totally random which type you get). The Roc version of the Mimic has the highest defense, so physical attacks might not work too well, although it has really low magic defense.
Mimics have nothing to steal and do not drop anything, also yielding zero AP. However, Mimics are one of the best sources of Gil in the game, as they always drop 50,000 upon defeat (100,000 with Gillionaire on an active character’s weapon).