User with this ability either is or can transform into an Extraterrestrial/alien: intelligent life forms that are of extraterrestrial origin. Given the near infinite variety of possible extraterrestrials, even only the ones in fiction, a user can conceivably use nearly any power.
Gain the form of beings from fables.
Posses any special abilities of cryptic creatures.
The Royal Four (Roswell) have this ability due to their alien ancestry
Despite looking like a human, The Doctor (Dr. Who), is an alien called a Time Lord.
The Asgard (Stargate franchise) were a benevolent, extremely advanced race from the Ida galaxy that visited Earth on many occasions. They used the cover of primitive gods in a similar manner to the Goa'uld, leading to the development of Nordic mythology.
The Goa'uld (Stargate Franchise) are a race of sentient parasitic beings that take over hosts.
The Wraith (Stargate Atlantis) are a vampiric hive-based species that harvest the 'life-force' of other humanoid beings for nourishment through suckers on their right hand palm.
Unas (Stargate SG-1) is a race named by the Goa'uld that translates to 'the first ones' or 'the first race' or more specifically, the first humanoid race that they used as hosts before Ra discovered Humans on Earth.
Cole/Daggon (Tracker) is a member of a species of energy-based extraterrestrials known as 'Cirronians'.
The Blancmange (Monty Python's Flying Circus) are a race of extraterrestrial, sentient blancmange that turn the UK's population Scottish so that they could win wimbledon.
The Legion of Super Heroes (DC Comics) is a 30th Century inter-galactic team of super-powered young adults. This cast expands to include dozens of members with a diverse array of powers and personalities.
Saturn Girl (DC Comics) hails from Saturn's moon Titan, where everyone is a naturally born telepath.
Garth Ranzz (DC Comics) also known as Lightning Lad or LiveWire hails from the planet Winath, where twin births are ubiquitous, accounting for almost all natives of the planet.
Cosmic Boy (DC Comics) hail from the planet Braal, where everyone has the natural power to manipulate magnetic fields
Lar Gand (DC Comics) better known as Valor or Mon-El.
Star Boy (DC Comics)
Andromeda (DC Comics)
Shrinking Violet (DC Comics) hails from the planet Imsk, where everyone has the natural ability to shrink down to sub-atomic sizes.
Kal-El/Superman (DC Comics), a Kryptonian, is perhaps the most well-known alien today.
Kara-Zor El, Supergirl (DC Comics) the Girl of Steel is also a Kryptonian.
Power Girl (DC Comics), another female Kryptonian.
Kryptonians (DC Comics)
The Guardians of the Universe (DC Comics)
The Burning Martians (DC Comics) were the dominant life form on the planet Mars before being genetically altered by the Guardians of the Universe.
Green Martians (DC Comics)
White Martians (DC Comics)
J'onn J'onzz, the Martian Manhunter (DC Comics) Mars's greatest son.
Tamaraneans (DC Comics)
Starfire (DC Comics) is an alien princess from the planet Tamaran with powers of flight and energy projection.
Daxamites (DC Comics)
Dominators (DC Comics)
Braalians (DC Comics)
Carggites (DC Comics) have the natural ability of Bio-Fission allowing the split their physical form into three independent bodies.
Kilowog (DC Comics) hailed from the planet Bolovax Vik, which was one of the most crowded worlds in the universe; it had sixteen billion residents with a communal mind.
Icon's (DC Comics) lifepod altered his DNA so he would resemble a normal human being, thus enabling him to blend among Earth's natives.
Mongul the Second (DC Comics)
Mikaal Tomas (DC Comics) better known as Starman
The Sinestro Corps (DC Comics)
Atrocitus (DC Comics)
Skrulls (Marvel Comics)
Kl’rt, The Super Skrull (Marvel Comics)
A Badoon (Marvel Comics)
Dire Wraiths (Marvel Comics) a magical offshoot of Skrulls
The Shi'ar (Marvel Comics)
The Shi'ar Imperial Guard (Marvel Comics)
Techarchy (Marvel Comics) a techno organic species
Eclipse the Darkling (Archie's Sonic the Hedgehog), a black arms alien.
Ceneca-10050 (Archie's Sonic the Hedeghog), a bem alien.