The next mythical Pokemon, Marshadow, has been known since November, when Pokémon Sun and Moon’s Z-moves were revealed. This new teaser video means Marshadow will finally be available soon.
Dataminers pulled out Marshadow's vitals long ago, though the creature has only now been formally announced. It's one of seven Generation 7 legendary and mythical Pokémon. Bulbapedia lists Marshadow as a fighting/ghost type.
The last one, Magearna, was made available to players through a QR code in December. Marshadow may be offered in the same way. Indeed, a news release touting Marshadow’s appearance said it cannot be captured in normal gameplay.
Above is Marshadow's Z-Move, the "Soul-Stealing 7-Star Strike.” Below, Marshadow appears in the latest trailer for the next Pokémon movie, I Choose You!, due to launch in Japan in mid-July.