An alien, or more accurately extraterrestrial, is any life-form originating from beyond the planet Earth.
- Greys, a race of aliens with mysterious ties to the planet Earth and its history. These aliens are sometimes also referred as Reticulians for their supposed planet of origin, Reticulum, although abductee Cassandra Spender implied the importance of the constellation Cassiopeia to their origin. The greys include the colonists, although not every grey appears dedicated to colonizing Earth. Note that long-clawed aliens appear to be just a different stage in the life cycle of a grey. In recent decades, some of the greys attempted to recolonize Earth after an ice age forced them to leave; it is stated that they arrived here "millions of years ago" and "walked the Earth before the dinosaurs" in one form or another. Greys in their early stages of life look like yellow and green muscular humanoids with reptile like skin, clawed hands, large black eyes, slits for a nose and a mouth full of sharp teeth. After exposure to immense heat, they transform into an adult grey, which is about the size of a human child, whereas the immature form of a grey is as big as an adult human. They retain most features of the immature version except they lack muscular bodies, green and yellow reptile-like skin, sharp teeth and claws on their hands. Instead, they have frail bodies that are smoother and grey in color with larger heads in proportion to the rest of their bodies. (It should be noted that it is unclear if the greys and the bounty hunters are the same species. In some instances, the bounty hunters are referred to as a distinct race from the colonists (TXF: The Truth). Other instances seem to suggest they are one in the same (TXF: The Unnatural).
- Faceless aliens and alien bounty hunters, shapeshifting aliens who may be greys themselves. Faceless aliens are free of the black oil (see below) and have mutilated themselves, sealing all openings in their faces, to prevent it from infecting them.
- Purity (also referred to as "black oil" or "black cancer") is an intelligent alien virus that is the "life force" of the colonists. The virus can enter a human body to control it, up to and including control of the host's fine motor functions and speech that can deceive even those closest to the host, and can also emit lethal levels of radiation from the host's body when the alien intelligence chooses to do so, which does not appear to harm the host. Under different circumstances, the virus can incapacitate the human host and incubate an immature grey (a long-clawed alien) inside its body.
- Kindred: The Kindred look and act like a group of separatist religious such as the Amish, but it is implied they are really gender-changing aliens. They apparently lived on Earth until Mulder and Scully learned too much about them, causing them to flee, leaving strange indentations in corn fields that resemble the outline of flying saucers.
- Martians: Martians are gas based lifeforms native to Mars. They can travel in space without ships and are able to possess other creatures such as humans. Besides their gaseous bodies, they have faces that look like twisted human faces, which they can even form when possessing a human. They are known to sabotage human space flights, and one possessed Marcus Aurelius Belt during a space mission when he was an astronaut. However, Marcus managed to throw himself out a window, killing himself and the Martian.
- Super soldiers : Also known as "human replacements", super soldiers are the result of the alien virus incubating inside a human and literally creating a duplicate of that person. The gestation process makes the subject appear to be dead, with all of their tissue in a state of decay despite brain activity and other functions continuing. The affected subject then sheds their skin (quite literally) and ceases to exist, with the new, physically identical entity being an alien. The alien replacement exhibits unusual traits, including near-invulnerability, the capacity to regenerate their bodies from almost any injury, the ability to breathe underwater (or simply survive without oxygen) and superhuman strength. It is unclear if the alien replacements retain the memories of the humans whose forms they assume, though this seems to be the case with several individuals. It has also been state that they have some connection to a government project to create "super soldiers", hence the misnomer. These soldiers were possibly tested during the Gulf War. However, the established fact of them being extraterrestrial in nature, and the unreliability of the sources whom provided the super soldier information, makes it unclear if there was any truth to that particular angle.
- Hybrids: Humans who have been, in some manner, enhanced with alien genetic material. The Syndicate spent decades trying to create human-alien hybrids, experimenting on a number of different individuals using slightly different methods. Some of these experiments were carried out using the knowledge obtained from German and Japanese scientists who came to the U.S. after world war II. This effort involved a number of different activities, from alien abduction to the creation of human-alien fetuses from which stem cells and tissue would be harvested for further research. (TXF: Essence) While a number of these experiments were done at different points in time, the only known success was Cassandra Spender. The alien colonists required this hybrid to proceed with colonization. (TXF: One Son). It was said that the new race of alien-human hybrids would be a slave race. What exactly this would have entailed, however, was never elaborated on. When the Syndicate was destroyed in 1999 at the hands of the alien rebels, some of these experiments ceased, but other attempts to create human-alien hybrids continued. (TXF: Essence) Some projects involved experiments on women in an effort to create alien-human infants. At least once such infant was produced, and possibly more. (TXF: Per Manum). There were other ways in which a person could become an alien-human hybrid. Fox Mulder apparently became one at some point in 1999. The Cigarette Smoking Man declared that Mulder was one, and that he contained genetic material that would allow humanity to survive the coming plague. How exactly this happened is unclear, though it seems to have involved both Mulder's exposure to the black oil virus, and the influence of the alien spacecraft found on the Ivory Coast. (TXF: The Sixth Extinction).
- Clones. Duplicates of existing humans, used for various purposes by the Syndicate and the alien colonists. Notable clones included the Gregor series, the Samantha series (based on Samantha Mulder), several clones of a young blonde boy, and clones of several young men. At least one possible clone, Emily, may have been a clone of Dana Scully. The clones tended to be laborers and workers, with some working on various scientific tasks for the Syndicate, while others were simply engaged in manual labor. Clones were not simply human duplicates. They were also hybrids, containing alien DNA. Due to this, they often bled green blood, similar to that of the bounty hunters. At least some clones seemed capable of recognizing the bounty hunters, even when disguised.
Known sentient aliens that appear human
Other alien creatures
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