"The official Nintendo Dream magazine recently conducted an interview with Gamefreak’s Ken Sugimori, Shigeru Ohmori (OR/AS game director), Shigeki Morimoto (game developer), and Kazumasa Iwao (OR/AS map designer). In the interview, Ken Sugimori revealed that the reason Mega Flygon doesn’t exist is because Gamefreak was experiencing, well, artist’s block!
Sugimori: For Aarune, he needed to have a Pokemon that could use the moves Fly and Secret Power, which are perfect moves for finding Secret Bases. And the only Hoenn Pokemon that wasn’t used by a key character that fit that criteria was Flygon. It does not Mega Evolve, but I really like Flygon.
Interviewer: Key characters often carry a Pokemon that can Mega Evolve, but Flygon is an exception, right?
Sugimori: Flygon has had the potential to have a Mega Evolution since XY, but we were unable to complete a design and so it was dropped from consideration."
[url=http://pokebeach.com/2015/04/mega-flygon-excluded-because-artists-blockv8k]source[/urlv8k]