There are several Pokémon that you will be able to encounter throughout the year during specific events in Pokémon Go. During the Halloween Mischief event from October 15 to 31, Galarian Yamask will appear as a lucky wild encounter, raid battle, or you may be able to hatch it from a 7 km egg given by a friend. Although you may have caught this Pokémon when it was first released, what are the chances of finding the glossy version of Galarian Yamask in Pokémon Go?
Unfortunately, Galarian Yamask's shining form was not released on Pokémon Go. You can encounter this Pokémon in a number of ways, but you don't have a chance to catch its shining version.
Based on when this Pokémon was released last year, we should expect this bright form to be released very soon. Niantic typically interrupts a Pokémon's release by making the Pokémon debut and then introducing the Shiny Pokémon during a different event about a year later. For example, during this year's Halloween Mischief event, Spinarak will receive its brilliant version. Although Spinarak has been around for much longer than Galarian Yamask.
While the standard form of Yamask has a brilliant version in Pokémon Go, the Galarian Yamask version does not, and you shouldn't be looking for it. If you want to fight Galarian Yamask in a raid to have a better chance of encountering his shining form, we recommend that you save your raid tickets.