Obviously, if you'd rather not be spoiled on upcoming, possibly legendary event Pokemon, you should stop reading now.
Pokémon news website Serebii reports that the latest issue of Japanese magazine CoroCoro has details on the upcoming Pokémon movie, which is reportedly called Cocoon of Destruction & Diancie. As we originally reported, Diancie is supposed to be a rock/fairy type—but we don't know how it'll be officially distributed just yet. A tie-in event with the movie might be likely given how Pokémon like Diancie have been distributed in the past. If you'd like to see official artwork of the Pokemon, you can do so here. Otherwise, here's an in-game refresher:
The other two Pokémon found in X & Y's code, Volcanion and Hoopa—are still 'officially' unaccounted for. Still, the fact that details about Diancie are starting to emerge means we might just have to sit tight and be patient. It might be a matter of time before we can all have the three Pokémon in our own Pokedexes without resorting to hacking—as hacking was the original method used to acquire these Pokémon last year.