Why are mimics spawning before hardmode?
You likely have a mimic statue in your dungeon that spawned on top of some wires.
http://terraria.gamepedia.com/Mimic
Chest statues, which spawn mimics, are sometimes generated on top of wires connected to a pressure plate from a trap. This can result in a mimic attacking and killing you early on in the game.
You can tell that this is the case by killing them and having them not drop any of the standard mimic loot (with the exception of mimic banners)