Nintendo hasn't yet revealed details on how players will receive Marshadow, the new mythical Pokemon.
Pokemon Sun and Moon has been steadily getting legendary Pokemon distribution events since its launch, allowing players to collect hard-to-find mythical Pokemon like Magearna. Now, Nintendo has revealed that the next mythical Pokemon will be Marshadow, a fighting/ghost type.
Nintendo revealed the new Pokemon but didn't post any details about it, such as its abilities or how we'll get it. According to Bulbapeida, Marshadow is a dual-type Fighting/Ghost Pokemon with the Technician ability . The area around the creature's face is similar to headgear and will "turn green when attacking or using its exclusive Z-Move." The bulbs on its arms can also turn green.
Marshadow is featured in the newest Pokemon movie: Pokemon: The Movie - I Choose You, so it would be safe to assume at least in Japan the Pokemon will be distributed alongside the movie, as it has done in the past. As for the US, a Wi-fi distribution event is likely.