Necrozma

Details: After capturing all the Ultra Beasts and completing the Ultra Beats Storyline, with the entire UB Taskforce going their own ways, you can find the Legendary Pokemon Necrozma randomly in the wild in the Farthest Hallow of Ten Carat Hill. This is the Pokemon that Looker claims he saw fly over the Melemele Island Sky.

Necrozma will be at Level 75 - the highest leveled Pokemon in the entire game - knowing the Moves of Stealth Rock, Iron Defense, Wring Out and Prismatic Laser. It also has the ability of Prism Armor - proving it wasn't an Ultra Beast after all - which reduces the power of Super Effective hits that Necrozma takes by 25%. It is a pure Psychic Type Pokemon.

After catching Necrozma, you can actually go tell Looker of your discovery in the hotel on Melemele Island (Route 2). He will inform you that Necrozma has the given appearance of an Ultra Beast, but is not such a Pokemon. Since Ultra Beasts are very similar to Legendary Pokemon but are not such, and since Necrozma resembles an Ultra Beast but yet has more power and differs from such, this confirms that Necrozma is a Legendary Pokemon.

(Many Pokemon Fans believe that Necrozma is the third piece to the so-called Alola Legendary Trio, with Solgaleo and Lunala. It doesn't have the same power of the two Cover Legends, but this leads many to believe that Necrozma isn't yet at full strength, much like how Zygarde was to Xerneas and Yveltal. Many believe Necrozma completes the puzzle nicely: both the Sun and Moon provide light to the world, and light travels through none other than prisms.)