>>25239230This is most likely true.
Though with the weird timing of the reveal(see
>>25235554) and GF obviously doing different things these days, I wouldn't say it's totally impossible.
True all "new" pokemon during generation were just forms of previous pokemon, which could be altered to be traded to the previous games. Battles displayed the sprite of their base form, until Gen 6 with megas that is. Idk why they didn't do that with ORAS-XY, instead of breaking compatibility with new megas.
Though there was one case with a trade incompatible new form: Spiky-Ear Pichu. Looking past the bullshit in-game excuse why it couldn't be traded, there's the technical reason. It couldn't revert to base Pichu, so they killed trade compatibility. Though, it should've been compatible between HS and SS games at least. Also, it should've been transferable to Gen V, I guess they didn't feel it was important enough to carry it over.
Point being, if Magearna is a new Gen 6 mon, it'll be like that Pichu and ORAS mega battles, and be incompatible with previous games.
This is all IF it is this "6.5", whether it's only Magearna added, or others, to "Z". It may just simply be a super early mythical reveal for Gen 7. The timing is just very odd.