Extraterrestrial Pokémon
The Pokémon world is not the only planet with Pokémon on it. There exist several Pokémon that originated outside earth that have migrated there. These Pokémon can breed and evolve with terrestrial Pokémon, and Poké Balls and items that work on terrestrial Pokémon work just as well on these. These Pokémon have adapted to the place where they landed on earth.
There are also myths involving Pokémon originating outside of the Pokémon world because of their resemblance and relation to extraterrestrial bodies.
Known extraterrestrial Pokémon
#386 Deoxys
A space virus fell to earth in a meteor, and was exposed to a laser beam. This caused the virus's DNA to mutate into the Pokémon Deoxys. Upon exposure to a meteorite, Deoxys's Forme will change. Deoxys has four formes, adapted for survivability in the wild, and its specialized formes have some of the highest base stats of any Pokémon. Its Attack Forme was adapted for combat against Pokémon native to earth, its Defense Forme was useful to protect itself from attack from Pokémon native to earth, and its Speed Forme was useful for fleeing. These formes in combination with its ability to regenerate lost body parts allowed it to survive in an alien world. Because of its exoticness, it is generally asocial.
#605 Riguree and #606 Oobemu
Riguree and Oobemu appeared 50 years prior to the events of Black and White Versions in the Resort Desert. It is unknown how or why they appeared, and they have not been seen since. This, and their having survived living in an alien world for 50 years, can be explained by their ability to rewrite the memories of the people and Pokémon around them. They survive hostile encounters by using its psychic powers to confuse opponents. Presumably because of the presence of Sandile and Zuruggu in the desert, they are no longer found there, and have relocated to high up in the Tower of Heaven and Unova Route 14, respectively, where there is not a {{type|Dark}] to be found.
Rumored extraterrestrial Pokémon
#173 Cleffa, #035 Clefairy, and #036 Clefable
The star-shaped Cleffa are rumored to have come to earth on meteorites. Wherever a meteorite lands, there is always a Cleffa near. Cleffa and their evolved forms, Clefairy and Clefable, dance in a ring on Monday nights, nights with a full moon, and on nights when meteor showers occur. Notably, the legends do not mention Clefairy or Clefable coming to earth on meteorites- only Cleffa. It still means that Clefairy and Clefable are extraterrestrial by extension, however.
#120 Staryu and #121 Starmie
Starmie are rumored to be extraterrestrials because they are star-shaped and they emit radio signals into space at night. Staryu gather at night and flash their red cores to communicate with the stars.
#646 Kyurem
An old legend in Kagome Town states that Kyurem fell to Giant Hole in a meteor, and began taking Pokémon and people from Kagome Town at night for food. It is for this reason that the people of Kagome Town constructed a wall to keep Kyurem out, and stay inside their homes at night. In the middle of Giant Hole is a small pond in a crater, and examining it will cause Kyurem to cry out and cause a blizzard in the area.