I got this Xerneas through a trade in exchange for a shiny Politoed (it had Damp).
The Xerneas is in a Master Ball, it's called "Nea", it is Adamant, it isn't shiny, it has 31 IVs in HP, Special Attack and Speed, and it knows Night Slash, Geomancy, Moon Blast and Horn Leech.
As soon as I saw it was in a Master Ball, I instantly thought it was hacked. And I also thought it was shiny at first, because I thought Xerneas's sprite looked like its official artwork (I've never owned one before). But it's not shiny. And I also presumed it had completely perfect IVs, but it doesn't.
So now I'm thinking that maybe it isn't hacked, and that it could be legit after all. Because apparently when you catch some legendaries, they automatically have 31 IVs in some of their stats. So could it's original trainer have kept soft resetting in front of the Xerneas you catch in game until it was Adamant, and then caught it in a Master Ball? (I'm sorry if this is too long.) :)