You cannot get a legit Shiny Arceus, you will need to get an Action Replay in order to enter a cheating code so that you can get the Azure Flute since the Azure Flute was never distributed. After getting the Azure Flute, you will need to enter into the Spear Pillar so that you can play the Azure Flute there so that it'll open up the way to the in-game Arceus.
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Pokemon Diamond, Pearl, and Platinum have Arceus.
You can only get Arceus by an Event.
Sinnoh.
...shiny Pokemon are always shiny... ._.
ARCEUS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!