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An idea i think would be neat is a pokemon that gets revealed on the official youtube channel as a cutemon or something that doesn't look that special, and it's marketed as so, but when you encounter one in the game it takes that illusion off and reveals its true form, which is a horrible, disfigured creature, like a bird with a bony face, human teeth, and claws on its wings or something, and no one ever mentions it because they're all under the illusion and seeing the cute-ish form. And on top of that it wouldn't even be a legendary, it would just be a midgame shitmon single stage like kecleon or girafarig.
Another idea on the same "not as marketed" vein would be a shitmon that jumpscares you out of nowhere in a forest. You specifically wouldn't be walking in the tall grass, it would appear in a completely random location and the start of the wild battle theme wouldn't play, it would just show the mon's face up close with its cry and the battle would start as if you were in the middle of it already.