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Disguise/Disguise clone. Could be called Imitation.
>Why
Well it is the "Imitation Pokémon", and is known to hold its pose as a tree until agitated. Hitting it could be like breaking Mimikyu's own disguise.
If you wanted to make it a worse-Disguise to justify thematic concerns then you could just limit it to Status moves (and Grass-type moves lamo) until it is hit.