They announced a second mega_mewtwo, there's now a mega_mewtwo_x and mega_mewtwo_y.
The originally announced form is the Y one.
See reference here: http://www.pokemonxy.com/en-us/pokemon/mega_pokemon/

Agreed. This alias needs to happen.

I guess an alias is fitting, much better than buttheaded_pokemon_to_be_named anyway

Also in cases where an alias is already present, please let me know so that I may delete it easily