Just thought I'd let you know, even if a Pokemon matches all the information given on Serebii/Bulbapedia/wherever, it can still be hacked. Good hackers have access to all this information as well, and can input it in when creating a Pokemon to make it appear legitimate. Since the shiny Arceus was given to you by a complete stranger, it is almost assuredly a hack. I hope you didn't trade anything good for it :/