When in 2013, Nintendo 3DS released Pokémon X and Pokémon Y, we all assumed that, as had happened in previous generations, there would be a third one to complement them. In fact, until that moment only Black and White had eliminated it, and that was because they had a second part planned. With the combos of Red-Blue-Yellow; Gold-Silver-Crystal; Ruby-Sapphire-Emerald and Diamond-Pearl-Platinum, everything seemed to indicate that Pokémon Z was on its way… but it never came.
I don’t choose you!
In fact, three years later, Sun and Moon arrived, which also did not have an alternative version, and it became a cliché of the franchise. At least for a while, Game Freak was considering making Pokémon Z, or that’s what we gather from the hacked files: we would return to Kalos and see the Legendary Zygarde with a new form.
In short, there were plans on the table, but in the end, they decided there would be no continuation of any kind: neither a third version, a DLC, or a sequel. In fact, it probably didn’t go any further because there isn’t a single mention of Z, at least in the documents leaked so far. Anyway, Kalos will return in the upcoming Pokémon Legends: Z-A, which may serve as a spiritual successor a decade later.
In the end, everything seems to indicate that Game Freak used the complete form of Zygarde, but did so in Sun and Moon. In fact, fans always found it strange that it appeared in Alola, a region where it didn’t even appear for the first time. This could be the explanation: since it was created for Z, they decided not to let it be forgotten. In Pokémon, as in stew, everything is used.
That said, it may also be that none of this is official. In the end, a handful of inconsistencies have slipped into the gigantic leak that had nothing to do with reality. There is still much more to discover.