Between the Pokémon anime that's been steadily airing since the '90s and the mainline video games running parallel to them, there has been a great deal of lore revealed about the world's powerful Legendary species. The Kanto region showed fans Ash's adventures encountering Pokémon like the Legendary Birds — Zapdos, Moltres, and Articuno — as well as the mythical Mew and the artificial Mewtwo it spawned.
Source：https://www.cbr.com/pokemon-most-powerful-legendaries-based-on-lore/
