Yamask is an October-exclusive spawn in Pokémon GO. Much like Delibird only pops up during the Holiday Event, this Ghost-type Pokémon from the Unova region can only be found during spooky season. However, rather than just during the end of October Halloween event, Yamask has already made its return to Pokémon GO. Here's where you can find it now.
Currently, Yamask can be found (very rarely) in the wild. It came back October 1st at 1 PM Pacific and, as a personal observation, I have seen one so far. However, it is currently able to be encountered through a Pokéstop task. If you are able to find the "Hatch an egg" task, this will lead to a Yamask encounter… just not during the Fashion Week event, as Smoochum has taken over that task. Outside of the event, though, this is an October task that will be here for the month, giving players a shot at getting this rare Generation Five Pokémon.
It is likely that Yamask will feature a boosted spawn rate during the upcoming Halloween event. This annual Pokémon GO event spotlights Ghost-type and Dark-type spawns. While Yamask wasn't exactly as prevalent as common ghosts like Gastly, Duskull, and Shuppet during the event, it was certainly appearing at a fair rate. Yamask does, however, have the full odds shiny rate of one in approximately 450, though, so if this rare Shiny is one that you want to add to your collection, the only way to better your chances is to grind.
Yamask has very interesting mythology that is more tragic than most species. When it was first rolling out, Niantic tweeted a teaser through their @PokemonGOApp account, writing:
Reports show that each of these Pokémon carries a mask that used to be its face when it was human. Sometimes they look at it and cry…
So if you see a wild Yamask chilling on the nearby, maybe go find it. It sounds like it needs a friend.