I've seen one in a Link Trade, so I'm just wondering.
Yes, it’s always possible for really any Pokémon to be hacked, but if it has the right OT, ID, encounter info, etc. as the one listed here, then it could be legit. That doesn’t necessarily mean it is, it just makes it impossible to tell apart from a legitimate one.
Technically speaking it's possible for pretty much anything to be hacked... Japan did, however, have a shiny Arceus event back in 2015. Try going over the information about the event here to see if it matches the one you saw.