I will try to keep this short also I don't own this theory, I just found it. I haven't seen anyone talk about this before so I thought of sharing.
*Mega Charizard X*
So we know that mega stones are morphed evolution stones that formed during the great war of Kalos. But at that time Mewtwo didn't exist. We know that Mewtwo was a human creation of Mr. Fuji(I will prove that also). So the creators also tried to make mega stones and made two stones i.e. x and y.
Now we know that except for Mewtwo Charizard is the only Pokemon to have two mega stones. Mega Charizard y is a generic mega evolution while the mega Charizard X is kind of some different thing. We saw in Pokemon origins that Mr. Fuji gave red the mega Charizard X key stone. So this proves why mega Charizard X exists and also the fact that Mr. Fuji made Mewtwo.
Also in the origins movie Mr. Fuji was the one who knew all about Mewtwo while telling it to her granddaughter.