Extraterrestrial Pokémon are Pokémon who come so from Space or do not live on Earth's surface.
The Pokémon world is not the only place in its universe that Pokémon exist. There exist several Pokémon that originated somewhere through space and have migrated there, as well as those who reside in their own dimensions. Many of these Pokémon can breed with terrestrial Pokémon and evolve in the same ways. Poké Balls and items that work on terrestrial Pokémon work just as well on these. These Pokémon have apparently 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, and in some canons, there are characters who speculate non-earthly origins for all Pokémon.
The Pokémon known as Staryu and Starmie are often stated to be extraterrestrial Pokémon due to their star-shape and multi-colored core (two traits that are shared with other extraterrestrial Pokémon). Their cores send radio-waves to other planets in space. Staryu are also known to gather in large groups at sea, flashing their red cores to the sky, as if signaling morse code. They are known for their remarkable healing properties that allow them to regenerate quickly.
Mew is said to be the ancestor of all Pokémon. Due to this, and the number of extraterrestrial Pokémon that have been discovered, it is possible that Mew inhabit other areas of the universe, and it is equally possible that they originate from a world other than Earth. Mew have been shown to be capable of flying through the vacuum of space, as displayed in the original Pokémon Theme.
The Pokémon known as Cleffa, Clefairy, and Clefable are known to come to Earth with the use of meteors and spaceships that often crash-land into the Earth's surface. The Cleffa line is also known for their star-shape. They are also known to gather Moon Stone from their homes in crash-sites, often in large quantities. Clefairy are known to dance in circles on certain days (Mondays (Monday can be translated into Moon Day), full-moon nights, and meteor-shower nights). They are shown to have an intelligent mind as they are capable of creating spaceships, runways, and are able to signal others for takeoff.
Lunatone and Solrock
The Pokémon known as Lunatone and Solrock, despite obviously looking like a moon and sun, are said to come from space and use psychic powers to give themselves power from both lunar and solar rays. They are both known as the "Meteorite Pokémon" and are said to have been discovered in ancient crash-sites.
The Pokémon known as Rayquaza is stated to live in the ozone layer, which is close to space, but it occasionally comes down in times of crisis or disaster.
The Pokémon known as Deoxys is stated frequently in the PokéDex to have originally been a space-virus that had attached itself to a meteor from space. Laser beams are said to have greatly mutated the virus into the shape of Deoxys, giving it power and life. When exposed to different kinds of meteors, Deoxys can change its form into its Attack, Defense, or Speed forms. In Pokémon Omega Ruby and Alpha Sapphire, Deoxys is shown to be encased in a meteor, attacking the player once its meteor shield is broken.
The Pokémon known as Elgyem and Beheeyem are known to come from space. When crashed onto Earth, they are shown to have intelligence as they are capable of making or telling other to make them spaceships. Beheeyem are also known to pilot spaceships that are working. They were first discovered when crash-landing into desert. It is also based on Area 51 aliens.
The Pokémon known as Kyurem, according to old myths, fell from a meteor that crash-landed into the Giant Chasm. The tale tells of a monster from space that roamed towns, bringing cold winds and would eat people it had taken away.
The Pokémon known as Minior is often seen on meteor-shower nights, where thousands of them fall from the sky. Once they fall to the ground, they break out of their shells. Once they do this, they rise back up into the sky. If one if unable to rise in to the sky for a certain period of time, it will die.