The mimic chests look like ordinary chests but they are a type of monsters. In Dark Souls3 you will find 13 of them. Mimic chests will be able to swallow you and do severe damage with it but you can spot them before. If the chain on the right side of the mimic is curved inwards you it isn’t the mimic one. If it’s mimic chest the chain will be curved towards you. Besides that you can hit with sword every chest to be sure.
The mimic chests will drop rare items after killing them. Also you will be able to obtain the Symbol of Avarice but it’s not guaranteed. Unfortunately there is a possibility that even after killing of all 13 you will not get it. The best way to do it is to get the item discovery as high as you can before you open one of the mimic. You must find any mimic chest, and then put him down with the Undead hunter Charm. After that he will open his mouth and you are able to take an item off it. If it is not the Symbol of Avarice you will quit your game and start again. You should take into account that any mimic chest can be killed only one time, they won’t appear again.