I believe I saw that it comes after Magearna in the Pok?dex which means it's a Mythical Pok?mon. I find it comically ironic that so many fake leaks almost sort of hinged around the idea that Marshadow was going to be this epic legendary with a bizarre Type and act to round out a trio with Lunala and Solgaleo.
I feel if there was another forme to this guy, the dataminers would have found it, just like they found both forms of Wishiwashi and all forms of Oricorio. With this in mind, I feel Marshadow will simply be like our more recent Mythical Pok?mon, which only sport a backstory which serves to promote a movie rather than have any sort of impact in-game.
As for its Type, since I'm not too impressed by the design, I'm fairly indifferent to what it turns out to be.