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.
Superman (DC Comics) is perhaps the most well-known alien today.
Kara-Zor El (DC Comics) better known as Supergirl.
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.