Mimics are monsters that appear in every area of Vagante. They drop an item and some gold after being defeated and can be differentiated from normal chests by looking at the lock on the sprite. Compared to the normal chest, Mimics have a slightly redder tint, and the upper half of the lock is lighter.
They will stand still until hurt or a player comes close enough.
To notice the Mimic, there is one precaution you should take: look at the lock of the chest. When the Mimic is disguised it looks like a common chest, but it has a slightly redder tint. Mimics have fairly low HP, so getting caught by them isn't a big issue.