Mew

Mew, Psychic-type legendary Pokemon Mewtwo Strikes Back, Mew made a couple cameo appearances at the start of the movie. A Mewtwo was cloned from Mew's DNA, extracted from Mew's eyelash. Later, Mew started appearing around New Island while Mewtwo was testing his starter Pokémon clones. Mew followed Team Rocket into the main part of the island so it could stop Mewtwo from destroying the world. After the end credits of the movie, Mew is shown flying to the mountains. In Lucario and the Mystery of Mew, another Mew first appeared as a Taillow, landing on Cameran Palace and then transforming into Treecko. It then transformed into an Aipom to play with Ash's Pikachu.

Ash and the others released their Pokémon to play with what they thought was an Aipom. After taking them to an attic, Kidd Summers sent her two Weavile after it. To escape, it teleported itself, along with Pikachu and Meowth. It then transformed once again into a Pidgeot and flew the Pokémon to the Tree of Beginning. While Ash and the others were looking for Pikachu, Mew kept it entertained with toys. Mew also nearly sacrificed itself to save the Tree of Beginning. Lucario stepped in and was able to save Mew, but ended up sacrificing itself. Mew is believed to still reside at the Tree of Beginning. It is last seen playing with a Bonsly. (Source: Bulbapedia)