The Pokemon Company has announced that the newest Pokemon – the mythical Marshadow – will be available in Pokemon Sun/Moon soon. The existence of the Pokemon is something that the community has been abuzz with for a while now- this is the first official confirmation, and indeed, acknowledgement, that Game Freak has made.
Marshadow is expected to be a Ghost/Fighting type Pokemon- a killer combo that leaves it with very few viable weaknesses. Apparently it cannot be encountered in normal play, which seems to imply that it will be a Mystery Gift transfer- no battling and catching this one, it sounds like. But that may turn out to be incorrect- for now, we have no confirmation regarding the details on how to acquire this Pokemon, after all.