The alien appears as a humanoid creature roughly the height of an average human. It possesses four limbs: two arms which end in three-fingered hands with opposable (or at least prehensile) thumbs and two legs whose digits cannot yet be discerned. The alien has a large head that features two pitch-black eyes, pointed ears, a flat nose with two nostrils and a mouth with nine-pointed teeth. The alien's skin is a pale yellowish-green hue with dark splotches scattered about.
When killed with a headshot, the creature's green-colored brain matter can be seen going airborne, leaving roughly half of its cranium exposed. An unidentified blue-colored anatomical component is seen when its head or extremities are damaged. The alien is apparently capable of breathing within the earth's atmosphere. When fresh, the alien's blood appears to be a light green. However, the blood that is found adjacent to the crash site appears much darker.
The only alien that appears in the base game in the Commonwealth in 2287 is the survivor of a crash landing just to the east of Oberland station. The alien is in an unmarked cave just north of the crash site. If the alien is targeted in V.A.T.S., the display will show that its right leg is crippled. On sight, the alien will immediately attack the Sole Survivor. If killed, its body can be looted for an alien blaster pistol and 295-500 alien blaster rounds. Next to the alien is a human skeleton and a ham radio that is the source of the garbled radio beacon.
While possessing similar equipment, the alien found is noticeably different from the species encountered in Fallout 3'sMothership Zeta, with a much more rounded head and sharp teeth. However, this could simply be a graphical update or artistic revision, as seen with other things that appear in both games, such as mirelurks or feral ghouls. Furthermore, the loading screens refer to them as zetans, further connecting them to the aliens from Mothership Zeta. That, and the noise the alien makes sounds greatly the same as the zetans make in Fallout 3.
Despite possessing an alien faction, if two aliens are spawned near each other, they will fire on each other until one remains. This is caused by the NPC aggression being set to frenzied, which leaves the character unable to discern between friend and foe.
The corpse of the alien can be eaten if the Sole Survivor has the Cannibal perk at level two or higher. When eating, the alien's blood is red instead of green.
The alien can also be pacified using the Wasteland Whisperer perk.
Encountering the alien has no effect on the conversation while initially meeting Jack Cabot or Tinker Tom.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
The aliens resemble greys, beings commonly described in alien abduction accounts. These aliens have been commonly reported since the 1950s. Later, the Greys became more mainstream when science fiction began depicting them. These kinds of aliens have come to represent the "stereotypical" alien. The name zetan may refer to the star system Zeta Reticuli, a real-world star system seen in a number of science fiction stories, such as the Alien franchise. It was also rumored to be the origin of the aliens who allegedly abducted Barney and Betty Hill in 1961, which was the first widely publicized report of alien abduction.
End of information based on unverified behind the scenes information.