Throughout the game, you’ll have heard about the guardian deities–the tapu–that reside in the various ruins, and of course you also ran into one of them yourself: Tapu Koko. Now that you’re the strongest Pokémon trainer in Alola, the four tapu will finally deem you worthy of gazing upon their presence.
Which translates to: you can catch them! For reference, the four tapu are classified as Legendary Pokémon, although they’re not on the same level as the likes of Solgaleo, Lunala and Necrozma. Even still, they’re a lot stronger than most other Pokémon–and highly recommended to have in a team.