The 18th ???-type Arceus has been around in all 4th Gen games, as seen here, but since the ??? type no longer exists in Pokémon 5th Generation, and presumably forward, that form (And type) was entirely removed. As for what XanderO said, it's used to prevent errors in the game's logic, but since it's nearly impossible to actually hack an item (And if it is possible with an AR, all effects would be lost once the power to the system is shut off) without first using a rom, the form cannot be seen unless the game is hacked.