Details: After capturing Nihilego, you must report back to Anabel and Looker at the hotel on Route 8. They will thank you for you assistance, also informing you that the information on Nihilego has been sent to Wicke. Anabel will also tell you that she was able to pull some strings with her superiors, and that you will be able to keep all the Ultra Beasts you catch during your Missions.
Looker will look to celebrate by leaving and getting some food, but he will quickly rush back in with news of another Ultra Beasts sighting. This Time it has occurred on Melemele Island, so he and Anabel suggest regrouping at the hotel on Route 2.
After arriving at the hotel on Route 2, the Ula'ula Island Kahuna, Nanu, will be with Anabel and Looker. He is one of their informants, giving them and you details on recent Ultra Beast sightings. This Time around, the beautiful Pheromosa (exclusive to Pokemon Moon) has been spotted, and four of them no less. And it is up to you to catch all of them.
Pheromosa, the Bug/Fighting Type Pokemon, can be found in the wild in Verdant Cavern. Like Nihilego, it will appear randomly, but can quickly be found by the change in background music. Each Pheromosa will be at Level 65, all with the Moves of Triple Kick, Me First, Lunge, and Bug Buzz. It will have its Speed immediately increased as the Battle begins, and its highest Stat will be increased after knocking out each Pokemon on your team because of its Beast Boost Ability.
After catching not one, not two, but all four Pheromosa, report back to Anabel and Looker on Route 2 for your next mission.