What you see are the alternate forms of The Order Pokemon, #718 Zygarde. According to Serebii.net and CoroCoro, Zygarde is prettymuch the Cell or Frieza (from Dragonball Z) of Pokemon: Its base form is its Core (top-left), followed by its Cell below that. Next is its 10% Form (the dog-like one) and THEN the one you’re probably familiar with, which is its 50% Form. The one that takes up the next page is its Pefect Form.
According to the data released with next week’s CoroCoro, if Zygarde 50% can’t beat an opponent, it will transform into its Perfect form, which exceeds the powers of Xerneas and Yveltal COMBINED. To put it simply, Zygarde is Kalos’ Rayquayza (Emerald and the postgame Delta Episode in OmegaRuby/AlphaSapphire), Giratina (Platinum) and Kyurem (Black 2/White 2). All three got their own respective game that focused on them as we all know. Zygarde getting his shouldn’t be a surprise. Pokemon Z will likely be the “most complete” of the sixth-gen Pokemon games: It will have the Mega Evolutions introduced in OmegaRuby and AlphaSapphire in addition to presumably introducing even more Mega Evolutions.
I can also speculate how Zygarde’s story will be done based on how it was done in Hoenn: You’re introduced to Zygarde’s Core fairly early on by Professor Sycamore or Lysandre. You are eventually able to use its 10% Form for most of the game. In a twist, Team Flare captures both Xerneas and Yveltal. After they’re unleashed, Zygarde powers to its 50% Form and gets overpowered by the combined strength of Xerneas and Yveltal. Lysandre gloats, saying not even Zygarde can fight both Pokemon at the same time. Zygarde then powers up to its Perfect form and stomps both Pokemon.
I think you will be able to capture both Yveltal and Xerneas as well: Since it’s technically a double battle, you will have to defeat one to capture the other one. The one you don’t catch there you will be able to encounter during the postgame. You will hear from Professor Sycamore that there are rumors a super-powerful Pokemon has taken up residence in Terminus Cave (where you find Zygarde in X/Y). There you will find the Legendary you didn’t catch before.
I could also see some of the event legendaries being changed in Pokemon Z: Mewtwo (Kalos’ Unknown Dungeon) will be replaced by Darkrai, making it the second Pokemon after Deoxys in OmegaRuby/AlphaSapphire to be taken off the “Event-Only” list. I could also see Articuno, Zapdos and Moltres being replaced with three other Pokemon: If Nintendo was feeling generous, they’d make those three Keldeo (Fennelkin), Meloetta (Froakie) and Shaymin (Chespin). They would also replace the Kanto Starters with Tyrogue’s evolved forms (Hitmonchan. Hitmonlee and Hitmontop).
I got a bit carried away there but…yeah. Expect to hear about Pokemon Z sometime next year.