I wanted to know because I was going to transport it from Omega Ruby. It doesn't require a mega stone to mega evolve, so could I fill the item slot with a Z-Crystal?
While it is not known to evolve into or from any other Pokémon, it can Mega Evolve into Mega Rayquaza, but only if it knows the move Dragon Ascent and is not holding a Z-Crystal. Bulbapedia
Thankfully, Game Freak had enough common sense to not allow Mega Rayquaza to be able to use Z-Moves. You can only pick one of Mega Evolution or a Z-Move.
