Alien

Aliens, also known as extraterrestrials, are several species that have come to Earth for various purposes. Among them are the zetan aliens.

Background
The only known purpose of the zetans is implied to be experimentation and study on human subjects. After the mysterious disappearance of the USSA space capsule Clarabella 7 and the pilot astronaut Colonel Hartigan suspicions arose, and in 2062 the existence of aliens became internally recognized by government analysts, even though to the general public their existence was still covered up and believed only by "crackpot" groups such as the Quaere Verum.

Sources suggest that many of the Enclave's more advanced weapons and robotic systems were based on alien technology. Even toy manufacturers such as Wilson Automatoys weren't immune to these accusations and the United States government, which was said to keep its alien specimens and technology mostly in Area 51 (like a flying saucer that was later recaptured by aliens but crashed in California, instantly killing the two pilots). The Pre-War Olney Powerworks stored alien power cells near their tesla coil research.

The Brotherhood of Steel around 2161 and the Shi around 2241 encountered the bodies of aliens or their spaceships. By extension, the Capital Wasteland Brotherhood of Steel have a moderate amount of alien power cells kept in the Citadel's armory and the Brotherhood Outcasts also have some alien power cells at Fort Independence. Also, before 2277, an alien body of unknown species was discovered in Fort Bannister. And the Enclave recovered an alien blaster and some alien power cells in the armory of the mobile base crawler at Adams Air Force Base.

Some of the advanced human technologies were created by using the alien technology like the AI project number 59234: Skynet, created in the 2050s who manages the entire Sierra Army Depot, as well as the device blocking telepathic and psychic commands, the allegedly alien psychic nullifier.

Biology
Aliens are diminutive, hairless, yellowish-green-skinned humanoids with short, wiry limbs and green blood. Their heads are disproportionately large, with round, heavy-lidded, solid black eyeballs with no discernible iris or pupil; a thin, toothless mouth with green tongue, and two pairs of small holes where ears and nostrils would be. Aliens have no opposable thumbs, each hand having three long, tentacle-like fingers with large suction cups at each fingertip that allow them to manipulate objects. They breathe a similar atmosphere to humans, and (at least those on Mothership Zeta) subsist mainly on a diet of strange alien worms and squid. In the Commonwealth, an alien is seen, able to survive in earth's atmosphere, in a cave near their ship's crashed remains.

Description
They seem to have a strict society focused on work and progress, with no apparent concern for hobbies or personal pursuits. Their architecture is spartan and purely functional: for instance, their bed (called a stasis chamber) is a sterile metal capsule with no mattress, blankets, sheets, or other pieces of bedclothes typically found in a normal human bed. Also, no discernible means of entertainment can be seen aboard Zeta. However, the aliens, in their own right, appear to have some degree of emotion, since alien workers display obvious fear when confronted with armed assailants. In another case, the aliens are shown to have an unusual fascination with Giddyup Buttercup, as demonstrated in a captive recorded log onboard Zeta, of Sally being taken to the recording room and noticing a Giddyup Buttercup. She notes that upon her calling attention to it, the aliens become very excited, though she has no idea why they act this way. There is also a storage room full of them in the ship.

They view other species as laboratory animals, worthy of preservation and storage at best and vivisection and experimentation at worst. They speak in their own language of high-pitched screeches and wails. As such, communication between humans and aliens is supposedly impossible. However, linguistics specialist Holly Barrisford did make an attempt to communicate with the aliens during her abduction. Barring recording some brief biographical information, the aliens simply aren't interested in discussion.

They are incredibly advanced in their various technological fields, capable of spaceflight and technological feats equal to, and often beyond, that of even the most advanced human pre-War technology. They also possess technologies such as long-term cryostasis, powerful rayguns, genetic manipulation, teleportation and holograms. However, they are totally dependent on technology. Because of this, they are physically fragile, lacking in raw strength and ineffective in melee-based combat. Very few possess especially protective armor and clothing.

Aliens typically wear spacesuits, the color of the suit apparently indicating rank, and soldiers generally carry shock batons, disintegrators and atomizers. They prefer to attack their foes in large numbers and tend to have guardian drones with them.

Encounters

 * In Fallout, the Vault Dweller can encounter a device based on alien technology (the psychic nullifier) and a flying saucer with two alien skeletons.
 * In Fallout 2, the Chosen One can encounter two technologies based on alien technology, bio med gel and Skynet.
 * In Fallout 3, the Lone Wanderer can encounter a downed recon craft named Theta and a dead zetan. In the Mothership Zeta add-on, the Lone Wanderer can be abducted by zetans at the alien crash site and taken aboard a spaceship named Mothership Zeta.
 * In Fallout 4, the Sole Survivor can encounter an alien spaceship falling from the sky shortly after reaching level 20. Companions have unique commentary upon observing the crash. The crashed UFO is always located east of Oberland station. Following green bloodstains will lead the Sole Survivor to a hostile injured alien which can be looted for the alien blaster pistol. Animatronic aliens can also be found in Nuka World, modeled after zetans and utilizing their alien blaster weapon for lethal and non-lethal purposes.
 * In Fallout 76, the player character may randomly encounter aliens, the Flatwoods monster, and find alien blaster pistols. There are also alien props able to be located in Van Lowe Taxidermy.


 * In Fallout: New Vegas, with the Wild Wasteland trait, the Courier can encounter an unnamed recon craft with two zetans and an alien captain nearby.


 * In the Creation Club quest Over the Moon, the Sole Survivor encounters a second alien craft abducting a brahmin, along with two hostile aliens with unique weapons.

Crashed ships
Six alien ships are known to have crashed in the United States:
 * A flying saucer in a special encounter in California.
 * A recon craft named Theta in the Capital Wasteland.
 * Another recon craft that crashed and exploded on contact with the ground in a random encounter near the Lone Wanderer in the Capital Wasteland.
 * At the Citadel, there is information on a terminal (in the A-Ring) about another ship of unknown type codenamed "Palandine" that crashed in a heavily wooded area near Hagerstown, Maryland which the government was not able to reach.
 * A recon craft with three aliens near Horowitz farmstead in the Mojave Wasteland.
 * An event in which a UFO crashes to the ground south of the Beantown Brewery in the Commonwealth.

Robots and computers

 * Experimental weapons drone - Experimental security robot armed with a drone cannon Ex-B.
 * Guardian drone - Security robot armed with a drone cannon.
 * Support drone - Maintenance robot.

Vehicles

 * Flying saucer - Small ship with a saucer-shaped body used for fast operations that houses two pilots. The only known flying saucer is the one that crashed in California.
 * Mothership - Large ship dedicated to long-term space travel and the study of other species. It's generally located in space. It houses a large crew of aliens and is commanded by a captain. The overall shape of the ship resembles a colossal saucer with another small disc-shaped section attached to the bottom. Known motherships are Zeta and the unnamed mothership which arrives after Zeta's bridge is captured by the Lone Wanderer.
 * Recon craft - Small ship with a disc-shaped rear body used for stealth operations that houses only one pilot. Known recon craft is Theta, another which exploded and sprayed in the Firelance event, and a possible unnamed craft in the Mojave Wasteland.

Medical

 * Abominations - Biological weapons created from humans with alien DNA.
 * Alien biogel - Ingestible curative gelatin-like liquid which accelerates organic tissue's regeneration processes.
 * Healing archway - Care generator restoring health, removing all radiation, clearing any drug addiction and restoring crippled limbs.

Food

 * Alien squid food - Preserved squid-like creature.
 * Alien worm food - Preserved worm-like creature.

Other devices

 * Alien epoxy - Advanced liquid repairing agent for weapons.
 * Alien homing beacon - Teleportation beacon sent by a mothership to return quickly.
 * Drone control device - Remote control for alien drones.
 * Force field generator - Portable forcefield used for protective cover or blocking a passage.
 * Inertia suppression field - Personal protection shield technology.
 * Misc alien items

Appearances
In Fallout and Fallout 2, they only appear in the form of skeletons. The crashed recon craft Theta appears in Fallout 3 with the first dead alien, and the aliens are the primary antagonists of the Fallout 3 add-on Mothership Zeta. They also make an appearance in Fallout: New Vegas if the Wild Wasteland trait is active. If this is the case, then they are spotted in the northernmost part of the map near Brooks tumbleweed ranch. In Fallout 4 they appear during an event in which a UFO crashes to the ground south of the Beantown Brewery in the Commonwealth. In Fallout Shelter they appear during the quest Humans Like Us, where they use optical camouflage to look like male human beings.

Behind the scenes

 * The zetan aliens were designed to resemble "Roswell grays," stereotypical representations of aliens in pop culture and described in alien abduction accounts.
 * The name zetan mirrors the name of the star system Zeta Reticuli, a real-world star system seen in a number of science fiction stories. 