Yes, it's a Hall of Origins Arceus. being from a pokemon file doesn't make it hacked (persay, by certain definitions). The question is whether it's legitimately caught, or if someone used the action replay for both the wonder card and the IVs.
I suggest checking the catch-date. If it is a "random/high date", then it is probably RNGed. If not, it's probably not. :(
You cannot get a shiny arceus except by the unreleased mystery gift for G4.
I believe I recognize the OT, I think he's on Smogon, and he may have hacked the mystery gift option, but he wouldn't have hacked the pokemon itself. It's probably legitimately RNGed. The best thing to do is to google the OT, ID#, and pokemon name and see if you can find the thread it camefrom, or someone else on Smogon who has it (and is trustworthy). Where did you get the pokemon to begin w/?