Unlike previous generations where there are tens of Legendary Pokemon you can find within a game, there are only a total of nine different Legendary Pokemon in the Seventh Generation. These include Cosmog (though technically an Ultra Beast), Cosmeom, Solgaleo, Lunala, Tapu Koko, Tapu Lele, Tapu Bulu, Tapu Fini, and Necrozma. However, only eight of which can be obtained in your given game; the other Cover Legendary Pokemon must be traded for.
On the other, the creators make up for this missing Pokemon by allowing a player to construct their very own Zygarde, the only Legendary Pokemon from a previous generation found within each Seventh Generation Game. Plus, this Zygarde can be built to have the very New ability of Power Construct.