It's only theoretical and the evidence may not me be true to the games, but in the first movie we see Mewtwo create new Pokeballs, that can even capture a Pokeball, and the pokemon once captured going through DNA engineering device to be cloned, so it's not too far to say that Mewtwo is the one who created the Mega Stones for him.
Although the theory might ...
I believe this is a plothole, stemming from the discrepancies between Mewtwo's origins in the games, and its origin in the anime.
In the games, Mewtwo is not a clone of Mew, but actually its child: the journals in Cinnabar Mansion refer to Mew giving birth to it. Mewtwo was then subjected to "years of horrific gene splicing and DNA engineering experiments", ...
Current Pokemon on her team:
Goldeen (Belongs to Cerulean Gym.)
Staryu (Belongs to Cerulean Gym.)
Starmie (Belongs to Cerulean Gym.)
Horsea (Caught in Kanto.)
Psyduck (Accidentally caught in Kanto.)
Politoed (Evolved from Poliwhirl, which evolved from Poliwag, caught in the Orange Islands.)
Corsola (Caught in Johto.)
Gyarados (Obtained in Cerulean ...
Leon (world champ)
Lance (runner up)
Raihan (top 8 member)
Wallace (former hoenn champ)
Brandon (frontier brain)
We have a clear answer for this now....even though most of the fans will find this disappointing.
Leon (galar champ) is officially the strongest pokemon trainer in the anime now. In the anime he is ...
In the II generation games that takes place in the Johto region the league called... be prepared, the Indigo Plateau, together with that there's to be mentioned, in the games the Pokemon leagues called.... the Pokemon league. That to be said, GenII games are known to be continue for the 1st gen games, occurring on the same land two years after the first ...
Though the question already cleared I have a more creative way to answer it.
In the Anime, Ash' Pikachu learned Volt tackle in the Battle Frontier season ep05 (425) though not entirely learned (by the following episodes when used it's soon failing) by Ash orders Pikachu to use quick attack, though using quick attack Pikachu soon combine it's electricity in ...