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This rove beetle (second and third images, first is the termite it imitates), except instead of having a fake termite on its abdomen, it has a fake bug type of your choice, preferably something like Combee, Weedle or Durant. Obviously, it hides in the nests of these Pokemon to steal food.
Maybe even give it a pre-evolution with a shoddier, less intimidating disguise, which evolves with the Pokemon (Stapilfer -> Rovenous?) Dark/Bug, abilities are Illusion/Infiltrator, HA Disguise.