Apparently there's Mega Evolution in Pokemon Fire Red Origins

Pokemon 8794 Mega Buzzwole Pokedex: Evolution, Moves, Location, Stats

Pokemon GBA Rom With Gigantamax, Mega Evolution, Z-Moves, Dexnav & More!

New] Pokemon Game With Mega Evolution, Radical Red, Z-Moves, Alolan Forms, Dexnav & More!

ROM Hacking update: mega evolution has been achieved in gen 4 : r/pokemon

Pokemon Mega Origins Cheats

Now only 20 mega evolution left and nowadays we get random trainers with mega. So which trainer or trainer variant gets remaining mega. Feel free to discuss. : r/PokemonMasters

Blackjack Rants: Gotta Review 'Em All, Part #30: Mega Evolutions, Part 2

Pokemon 8197 Mega Umbreon Pokedex: Evolution, Moves, Location, Stats

Mega Stone - Bulbapedia, the community-driven Pokémon encyclopedia

Mega Stone in Pokemon Origins! : r/pokemon

Mega Stones Locations (Part 3) - Pokémon The Last Fire Red V4.3 (GBA)