Unlike previous generations where there are loads of brand new Legendary Pokemon you can find within a game, there are only a total of nine different Legendary Pokemon that were created and released during Seventh Generation. These include the likes of Cosmog (though technically an Ultra Beast), Cosmeom, Solgaleo, Lunala, Tapu Koko, Tapu Lele, Tapu Bulu, Tapu Fini, and Necrozma. However, only eight of these Pokemon can be found within the game, as the other Legendary Pokemon Cosmog can evolve into is a Version Exclusive and thus must be traded for.
On the other hand, the creators at Pokemon make up for this missing Legend by allowing a player to build their very own Zygarde once again with the help of Dexio and Sina, the only Legendary Pokemon from a previous generation you can obtain outside of an Ultra Wormhole. On top of this, the Zygarde you help create can be built to have the ability of Power Construct, allowing you to have Zygarde in its Complete Form during Battle.