This is all I could find:
http://www.gamefaqs.com/boards/696960-pokemon-y/68506259
It seems to me that most preposterous legendaries are from Japan, but I don't know. Use that link in combination with with whatever your legendary is and I think you've got you're answer to whether or not it is hacked. I tried this on ORAS with a 5th Gen Pokemon from Japan as well, and it said the exact same thing. What I did is went to the screen that shows you the Nature and Characteristic, and so it said "A fateful encounter seems to have led it across space and time to reach you from the Unova region. Ulitmately, I'm not sure, if you think it is do whatever
Source:
The link above and testing it out on my Japanese Event Genesect