If you want to see a ??? type arceus there is one on Bulbapedia: http://bulbapedia.bulbagarden.net/wiki/%3F%3F%3F_(type) or you could hack it onto your game (not meant to take seriously) and marvel at it during battle.
EDIT: I also found another bulbapedia page with the back sprite of the ??? type arceus: http://bulbapedia.bulbagarden.net/wiki/Pok%C3%A9mon_Diamond_and_Pearl_beta The arceus was first programmed into the beta of D/P.