Hyrule Warriors

[Depression Moon]

Are there any Majora's Mask characters in it?

No. But there is a Majora's Mask DLC coming.

DLCs announced thus far:

Master Quest Pack.
Twilight Princess Pack.
Majora's Mask Pack.
Ganon/Boss Pack.

The Master Quest pack is known to contain Epona as a weapon for Link. Contains a story mode for Cia, Volga, and Wizzro, explaining how they met, and offering upgraded weapons and heart pieces/containers for them. Contains "Master Quest" mode, which allows you to play with a harder difficulty (including one where no hearts spawn on the map). There's a rumored new Adventure Mode pack as well, but no new characters are believed to be added.

I don't really see how we can get a Majora's Mask pack without getting some characters from the game, though. Sure, we can maybe get Masks as another weapon for Link (adding in his Deku, Zora, and Goron transformations to his combos). But beyond that, you can't do a lot without bringing in some characters.

Epona as a weapon? How does that work; does he swing her around like a sword?

I take it you haven't unlocked all of Link's weapons yet. They're not all as straightforward as the word "weapon" may imply.

My guess is that Epona is actually going to work as a warhorse. Trampling, charging, kicking, biting. Definitely potential there. I suppose Link could just use his bow or sword for mounted combat as well.

I actually don't have the game.