I repeat again, that this is not common. I expected mega-flygon to be leaked by now more or less. Either it isn't gonna happen, or gamefreak did a design so incredibly awesome that if they leaked it it would blown the internet away.
Maybe there will be megas in the post-games ? I mean Mega-Gyarados was a post-game mega.. what if Flygon is one too.. but you're not the only who wants a Mega Flygon (and I want Mega Arcanine too)...