A mimic is a flesh-eating monster that looks like a chest:
In my fictional medieval fantasy world, they cannot shapechange. They have that appearance (and with their mouths and eyes closed, resemble wood-and-metal chests up to a casual inspection).
They usually have a wooden skin texture, and metal-like frames. They also can slightly change their textures and colors (even reproduce polished metal) to better fit in a colony of monsters, or with other chests around them, the origin of the name "mimic".
When a unaware adventurer comes to loot treasure from what appears to be a random chest, it springs and attempts to eat him. Quite successful that strategy is. Usually a human does not have the reflexes to avoid the fatal full-body bite from a mimic.
The monster only attacks if somebody attempts to open its mouth (lid) or attacks it. It can even be carried away and will remain inert.
In writing the origins of said monster, I am between two possible explanations (there could be a third that has already been discarded, that a mad wizard/god created them):
Mimics are monsters that evolved to look like chests, because that was effective in getting prey (adventurers and intelligent monsters).
People crafted chests to look like monsters (so would-be thieves avoid stealing for fear of being eaten).
Some wealthy merchants and aristocrats keep mimics inside their treasure vaults (or so thieves' legends say) to prey on thieves.
So far both explanations are good for me, in a dark fantasy setting. So, what one is the one with the least assumptions (and less suspension of disbelief), since we discarded "a wizard did it"??? (i.e. MAGIC is not a viable explanation).
Edit, because boldfaced and/or CAPS text is not enough:
There is magic in this world, but only small-fry stuff. No chimaerical creature creation magic is possible. Also, no gods walk the earth or do random stuff like making monsters on a whim. So intelligent design is out.