zygrde isn' new, and magearna is not a legendary
EIther way, regarding Tapu Koko, Its not a legendary. This is for two reason. We know it is owned by Hala. However, there has never been a pokemon that cannot be caught(technically, volcanion will become obtainable eventually). So we do know that we can also get it. That means that there needs to `be two. So it can't be a legend
1. Zygarde's Formes are new. And even if it's the species itself is not new it's still a Legendary yet doesn't have the Legendary filter on the site.
2. Magearna has been confirmed to be a Mythical Pokemon.
3. I guess this means Kyurem isn't a Legendary as it allowed itself to be commanded by Ghetsis. :P We know Hala is the strongest trainer on Melemele Island and that Tapu Koko is a curious Pokemon, on occasion it probably allows itself to be commanded by Hala (and maybe later the playable character) to battle someone if finds intriguing. "Why doesn't it battle by itself"? A Pokemon is at its strongest when battling alongside a person, also maybe battling alongside someone means it can't be captured.
4. As I said, there's been multiple Legendaries who have been treated like there's only one of them, specifically the Pokemon with godlike powers. Weather Trio, Creation Trio, Lake Guardians, Arceus, Regigigas, Forces of Nature, Tao Trio, Mortality Duo, and now Solgaleo and Lunala. Even Mewtwo and Genesect are treated like there's only one of them. It's called
story and gameplay segregation. They want these Pokemon to be special so act there's only one in existent though at the same time it's a game so want the player to catch and use them. Storywise, if you're acquired to catch a Legendary, I guess it's assumed the player character eventually releases it after their adventure (a part of the game the player doesn't see).
5. It's just been confirmed Zygarde Core & Cell, which are considered Pokemon, cannot be used in battle. Thus they can't be caught either.
However, as
Coronis said, they probably won't tell us anything more until the games are released (which I sort of hope they do. Tapu Koko they sort of needed to tell us about as we see it early in our adventure. However I would like to discover the other three guardians myself). No one is convincing anyone of their initial opinion so let's drop it.
It's great to see that Electric Terrain is getting somewhat of a buff with Electric Surge. The problem with all of the Terrains (Grassy, Misty, and Electric) is that they hardly have good distribution as it is (hopefully the new games change this), and the Mons that can learn them normally have better stuff to run, so they're all but useless. But having an ability automatically set it like the Weather abilities (hopefully we don't get Terrain Wars...) makes them better, even if just slightly. Electric Terrain is actually a great effect, as not only do you basically get an Electric Gem to your Electric-Type attacks, you also become immune to Sleep, which would actually be a pretty big deal in OU and Ubers depending on how good Tapu Koko is (I'm assuming it's going to have at least 580 BST like all of the Non-Cover Legends do).
Speaking of the Terrains, now that we know of one ability that automatically sets a Terrain, do you'll think there will be more "Auto-Terrain" abilities to follow in the future, such as Grassy Surge or Misty Surge? I'd hope that this is the case, as the other Terrains are genuinely interesting, but as said before, having to waste a moveslot and a turn that could both be used for something better in the long term is a waste. I would personally also like to see an item that extends the turn limits of these abilities, but this is borderline wishlisting as it is.
First I think we should keep in mind just because there's a Grassy & Misty Terrain doesn't mean they have to make an Ability for them. For one it wouldn't make sense for one of the Guardians to get an auto-Misty Terrain as Tapu Koko is part Fairy-type so why wouldn't it get it? Also, if all Guardians were to get an auto-terrain Ability, that would mean they'd need to make at least one new terrain and if they're doing that who says they can't make more? It was probably either planned to have Tapu Koko have the auto-Electric Terrain Ability or picked to be Melemele Island's Guardian because of it as we're familiar with Electric Terrain, for all we know the other Guardians have all new terrain moves.
On the freemium 3DS game Pokemon Shuffle, there were a few interesting Pokemon released - the new forms of Zygarde that have since been confirmed to be in the upcoming games, Ash Greninja, and something called "angry Pikachu" (whose ability is a massive damage multiplier). Certainly seems to suggest that at least Pikachu will be getting some sort of special buff/move. I'm wondering if perhaps the rumored "synchroevolution" doesn't so much entail merging Pokemon and trainer aesthetically like seen in the anime Greninja but rather is a buff of some sort to the Pokemon/its moves in a different way than mega evolution. And speaking of which, kind of strange they haven't mentioned it at all yet or revealed any new mega evolutions (or even showed mega bracelets), though I don't see them ditching the concept.
Or Angry Pikachu is just a special variant these side games can have. Also I think by now Pokemon Shuffle has released all the Pokemon there are, unless they start putting in the Gen VII Pokemon in early they'd need to start adding in special variants if they want to release new tiles.
