After defeating Ultra Necrozma and returning to The Alola Region, the Ultra Recon Squad will continue to allow you to explore Ultra Space with the help of their Legendary Pokemon. While traveling through Ultra Space, you may sometimes enter a White Wormhole that leads to Ultra Desert. This is the dwelling place of Pheromosa.
In order to battle Pheromosa, you will need to complete the strength puzzles that lie before you with the help of Machamp Shove (Poke Pager). Once you have completed them, you will be able to reach and battle Pheromosa. Pheromosa remains a Bug / Fighting Type Pokemon, always being found at Level 60 and knowing the moves of Triple Kick, Lunge, Bug Buzz, and Me First. Like all other Ultra Beasts, it can now be found in its Shiny Form. It will also respawn with each successive visit to this area.