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.)
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.