>>54840981>The first Pokemon to Mega Evolve is rayquazaThis is never stated. The closest "evidence" there is in this regard is an out of context statement by Bulbapedia that misinterpreted pic related.
Zinnia's grandma more accurately says that Rayquaza's appearance marked the beginning of the Draconid people that worship it. They still existed prior to it Mega Evolving and AZ showing up when it did means that it clearly wasn't all that new.