just wondering, like can you find it, is it coming as a mystery gift, or has it already came as a mystery gift?
Marshadow has not been released yet. However, like Magearna, it is a Mythical Pokémon, so it will be released in an event sometime in the future, probably this year (unless GF pulls a Hoopa and makes us wait until the next games.)
There was only one time in Pokémon Sun and Moon in which you could obtain a Marshadow: the Marshadow Distribution Event, in which you could by a card at various game stores depending on the country, and it would have a scratchable code that you could use to claim the Marshadow as a Mystery Gift. You also would have received its Z-Crystal. This is one of the several occurrences in SM/USUM of a mythical Pokémon only being obtainable through an event.
The only other legitimate way to get it was to trade for it, but you can't do online trading in SM or US/UM anymore as the 3DS servers were shut down on April 8, 2024.
You cannot simply find Marshadow as an in-game encounter.
Source 1: Pokémon Sun and Moon - Serial Code Events--Serebii Source 2: Youtube Video by Austin John Plays
We don't know at the moment. Maybe they might distribute him as a mystery gift in the future. The only mystery gift we've gotten in Pokémon Sun and Moon is the Munchlax event.
There is no official way of getting it so far, the only way to get it currently is hacking or using a cheating device.
