This actually IS possible. It was distributed in Japan forum August to the end of September. That being said, that's just the Pokedex entry, no one should believe it unless we see the Pokemon's profile itself :P
Holy crap.... I really should start playing Pokemon X and Y again. Or, should I just wait for Omega Ruby and Alpha Saphire and hope to get that sexy Beldum with Meta Metagross' megastone :/ Hmm, it's so hard to decide >.>
In the Glittering Cave you can find an area with a big room and the floor is really rainbowy and shiny I think thats where Diancie spawns, but idk, I found that out like 4 months ago xd