Duck Hunt Dog belongs to Nintendo and is very inconic, it was packaged with the orginal super mario bros, in fact this was a good selling game on the NES. I'm not a big fan of Duck Hunt Dog, but I personally wouldn't mind Duck Hunt Dog got in. It all just depends on its moveset, if they can create a good fitting moveset for Duck Hunt Dog, I'll be all for it, but if they create a really unfitting moveset, Dog Hunt Dog will probably be my next punching bag. Duck Hunt Dog certainly does have a chance, I would personally want a different retro character. They shouldn't prioritize newcomers over veterans, I would rather have a game with no newcomers than the characters as DLC.
As for the leak, when I first looked at it I this was 99.9% real, but when I looked your guys' comments, the I realized that this probably fake. Why would they exclude Mii Fighters, Wii Fit Trainer, Pac-man and the Paper Mario stage but show everything else? Why does Wario almost have the same look on his face in Subspace Emissary? Even if the Duck Hunt Dog's artwork is really good, I think this is fake.
The video is private, what did it show?