- 493 in the National PokeDex, Arceus is known as the Alpha Pokemon, or by some the God Pokemon. Arceus was recently shown in the 12th Pokemon movie, "Arceus and the Jewel of Life". Arceus is an event Pokemon, so they only way to get it without an event is Action Replay, which would not get you a legit Arceus. They just recently started events for this rare and powerful Pokemon in America, but they started a while back in Japan.
It is also said that in the Old Chateau if you follow the elderly ghost through the wall using an Action Replay, he will give you a strange fist plate that goes with your Key items. Then, if you go to Spear Pillar, you will fight a trainer with a POkemon that is strangely level 1000, not found in the pokedex known as Araceus. If you throw three Master Balls, using the Action Replay, you can catch it. But, if you save and reset the game, the Arceus will become the notorious Missingno, the sprite in the PokeDex when you have not seen a certain Pokemon yet. It's real name is not Missingno; it is ???. People just like to call it that. NOTE: I did NOT make this Araceus thing up; my friend told me he did it and it happened; and I told him Missingno could mess up his game andt hat he sshould release it.
Here is the Pokedex entry as well:
Diamond: It is described in mythology that this Pokemon shaped the world with it's 1,000 arms.
Pearl: It is told in mythology that this Pokemon was born before the universe even existed
Platinum: It is said to have emerged from an egg in a place where there was nothing, then shaped the world.