Treasure Chest Mimic
Mimcs are a class of mobs that hide their true nature, to ambush the adventurer. They appear to be a harmless object at first, but they will attack the player upon interaction.
Mimic Treasure Chests can be found in the temporary hideout of Karzov (non-persistent location) in the Blacklake District, in a shipwreck at Raven Cliff Beach and in the Sea Caves, the last two are both in Blackdagger Ruins. The beach is a persistent location, so to gather kills of Mimics, it is recommended to kill the Treasure Chest there and then wait for it to respawn.
There is currently a bug that will sometimes cause the chests to change their appearance and show their teeth before being interacted with, usually when other mobs nearby go into battle stance. However, they are not really attacking yet. This way, they are easy to spot and the adventurer can prepare for the attack, or chose to not interact with the chest at all to avoid the ambush.