Erron Black
The Outlaw
Also as a quick proponent to my last post, We have seen a lot of fake leaks saying ash greninja is in the game and every time one is disproven people still believe its existence.
As said multiple times before(For Megas, but it still applies here) just because we've yet to see anything/hear anything about it, doesn't mean it will/won't be in the game.
However, since Zygarde's forms are confirmed to be in-game, it's highly likely Ash-Greninja will be as well.