Aah! AAAAAhh!! Oh my god what if it's like this!!
That pokemon could be part of, if not related to Zygarde (like bees to a queen) and happens to be a completely brand new pokemon! #722 and possibly have more new pokemon following it to be added into an expanded Kalos, which now has bigger cities and more challenge and features or something? The game could be a dex-fixer like in Platinum, but megas don't exist until the time of the story, maybe near the end so Diantha still gets megas, and a bunch of previous generation pokemon are replaced in the dex by brand new ones!
That pokemon could be part of, if not related to Zygarde (like bees to a queen) and happens to be a completely brand new pokemon! #722 and possibly have more new pokemon following it to be added into an expanded Kalos, which now has bigger cities and more challenge and features or something? The game could be a dex-fixer like in Platinum, but megas don't exist until the time of the story, maybe near the end so Diantha still gets megas, and a bunch of previous generation pokemon are replaced in the dex by brand new ones!