After becoming the first ever Alola Region Champion and after the credits roll, the Post Game will begin with the Ultra Recon Squad needing your assistance. There have been reports of Ultra Beasts being found in Poni Grove, and they need you and Hau to battle and catch these ones in order for them to progress their research with the Aether Foundation.
Within Poni Grove will be the Ultra Beast known as Stakataka, three of them to be exact. All three will be found at Level 60, knowing the moves Iron Defense, Iron Head, Rock Blast, and Wide Guard. Stakataka is a Rock / Steel Type Pokemon, one that has extremely good defense and attack stats. After battling/capturing two of the three Stakataka, the four of you will leave Poni Grove. You can return at anytime afterwards to obtain the final one.