That looks like Arceus' Ground form, but you said it isn't holding an item. I don't know the full extent of what's possible with hacking, but I'm pretty sure that all of Arceus' different forms are stored in the code as separate Pokémon, so the OT could have been able to hack in Arcues' Ground form directly, without the need for the Earth Plate. If that's the case, then the hack check would absolutely have a problem with it. Hacked Pokémon usually don't have any negative side-effects on your game, since they often use completely legal values to make them indistinguishable from legitimate Pokémon, but such an illegal Pokémon like this could get your system bricked. I would release it.