From the Super Mario Wiki
A Treasure Pest is an enemy in Wario: Master of Disguise. It makes its first appearance in the Smithsnorian Museum and appears in all locations following it except for Sweatmore Peak. From afar, they look exactly like a red treasure chest, but when Wario draws near, it reveals its big eye and attacks him.
Treasure Pests randomly replace one red treasure chest (that is, one that does not contain a level-specific item such as a key, or that level's map) at the start of the level, preventing the player from winning that treasure; Wario will have to return to this level later in order to get the treasure the Pest replaced.
A devious mechanical trap made to look like a treasure chest. It yearns to put a hurtin' on all who draw near to steal the sweetness within.
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