Mimic

From Boktai - Wiki on Neoseeker

A Mimic is a deadly type of monster that is hard to find, because it disguises itself as a treasure chest. Normally when a player sees a treasure chest, they will go to open it; however if that treasure box is a Mimic, pressing A will activate that Mimic somewhat, and the Mimic will attack the player. They are not easy to get rid of as they jump around quite quickly, so be careful when facing against a Mimic.

Using a See All Nut will reveal whether or not the treasure chest is a Mimic. If it is, the chest will flash blue. It is best to fire at it or avoid it completely. If you are uncertain of whether or not it is a Mimic, you may choose to attack it first. If it takes damage, it is a Mimic.