From Bulbapedia, the community-driven Pokémon encyclopedia.
In each Pokémon game of Generation IV, all Pokémon have an unseen index number used to identify them in the game's ROM. All of them are in their National Pokédex order; however, the alternate forms of Deoxys, Wormadam, Giratina, Shaymin, and Rotom have their own index numbers. Numbers 501 to 507 were added in Pokémon Platinum and are empty in Pokémon Diamond and Pearl.
List of Pokémon by index number