Grey

Grey aliens, sometimes known as Zeta Reticulans, Grays, or simply Greys, are by far the most commonly reported species involved in UFOlogical cases. Named for the coloration of their skin, these sapient humanoids have gathered fame and attention from researchers thanks to such infamous incidents as the Roswell UFO crash of 1947 and the Barney and Betty Hill abduction case in 1961; and their image has found its way into popular imagination and mass media to represent the archetypal, flying saucer-using, memory-erasing alien visitor who abducts subjects to study them.

In actual Fortean theories, the relationship between Greys and other races such as Reptoids is usually more complex, although it's frequently believed that the Greys themselves might be both reptilian and genetically-engineered in origin, and that they're employed; or in some sources explicitly enslaved; to act as a researching workforce and do the other race's bidding. Conversely, alternate ideas abound, including some picturing the Greys as benevolent, time-traveling descendants of Humans whose main purpose is to monitor and guide the development of their ancestors.

Biology
In the majority of cases, the Greys have been consistently described as short, thin humanoids with grey and hairless skin and a conspicuously large cranium. Their eyes are large, black and almond-shaped, and their facial features are restricted to a pair of small slits in the place of the nose, and a small mouth. Their fingers are usually long and dexterous. Their bodies are proportionally small, and they don't appear to have genitals, which is noticeable due to the fact that they don't wear clothes. In some cases, they have been reported as capable of telepathy.

The Bellero Shield
In the early sixties, The Outer Limits presented the episode "The Bellero Shield" revealing a greyish type alien that may have been the inspiration for what Barney and Betty Hill thought they saw during their alleged abduction experience. The aliens that Betty and Barney Hill described, while under hypnosis, looked remarkably like the grey creature in "The Bellero Shield". And although the actual abduction predates the episode by three years, the hypnosis section that led them to remember the alien's appearance did take place just a few days after its airing. Since the Barney and Betty Hill case, Greys have most often been associated with the alien abductions, a trope that is often repeated in science fiction films and novels.

Solipsi Rai
According to the book Alien Races, by Dante Santori and Petro, the Greys are called the Solipsi Rai, a two billion year old race from the constellation Cygnus and is led by a prominent leader named Ymartyyn. The Greys are often confused with other races such as the Maitre and the Zeta Reticulai (the actual abductors of Betty and Barney Hill) due to similar appearances. The Solipsi Rai, are a peaceful and harmless race that have no colonies and focus more on the development of other planets and races. They do however have one powerful weapon to protect themselves from invaders and is capable of destroying entire ships. One of their ships is responsible for the Roswell incident on July 7, 1947. They have also stated that the Human race still has 645 options for the future of their planet, with the correct options depending on their ability to develop space travel.

In popular culture

 * The species is featured in the 2007 movie Night Skies, based on an actual abduction case which took place during the Phoenix Lights event of 1997. The creatures portrayed on the film appear like skinny Greys with telepathic abilities and organic-based technology.
 * The species is featured in the Doctor Who animated special Dreamland, engaged in war with the Viperox. In the Third Doctor novel Devil Goblins from Neptune they are given the name Nedenah.
 * The Silents, although a different species, are similar to Greys in appearance and might be related.
 * There is a species called Grays in Alienology, that differ little from the standard definition of the species.
 * The Arilou Lalee'lay from Star Control are perpetually-mischievous that are intended to represent the Greys within that universe.
 * The Asgard from Stargate are typical Greys.
 * In the third episode of Extreme Dinosaurs three alien Greys appear imprisoned in a government facility, only to be freed by one of the Extreme Dinosaurs.
 * The aliens from the miniseries Taken are basic Greys.
 * Salarians in Mass Effect somewhat resemble the Greys.
 * In South Park a frequent Easter Egg in the show is the presence of what they call Visitors, aliens virtually identical to the mainstream idea of a Grey in every way.
 * The Furon are most likely inspired by the Greys.
 * The Greys also make an appearance in the 2001 MuckyFoot game, StarTopia, in which they can run the player's Sick Bay facilities.
 * The Interdimensional Beings from Indiana Jones and the Kingdom of the Crystal Skull are basic Greys.
 * The Colonists; a.k.a. Reticulans or Greys; are featured as part of the major mythology of The X-Files series.
 * The Babylon 5 universe features at least three species designed after the Greys of popular culture: the Streibs, the Zeners and the Vree.
 * The cartoon Buzz Lightyear of Star Command names them Roswellians and shows an inversion of the classic story; in which Buzz and his team crash land on the Grey's homeworld.
 * Roger from American Dad is an obvious example of a Grey in modern media.
 * Paul from the movie of the same name is a very deliberate example of a Grey.
 * The What from The Tick disguised themselves as a type of grey when they abducted the shows titular character.
 * The Sectoid from X-Com are that universe's variation of the concept.
 * Barash from Star Trek: The Next Generation belongs to a Grey-like species.
 * Greys appear (under the name "Grays") in the second and third Ratchet & Clank games.
 * In the 2013 horror-science fiction film Dark Skies, the main villains are named "the Greys".
 * The Vargon from the American Chillers novels are essentially Greys.
 * Men in Black features two species - Baltians and Arquillians - which bear some similarities to this species. However neither fits well in the size factor, the former being much taller than how the typical Grey is usually portrayed and the latter being several times smaller.
 * The mature form of the Skrit Na from Animorphs.
 * They are featured as one of the inhabitants of Area 51 in the animated comedy Escape from Planet Earth. 
 * The aliens from the 1977 science fiction film Close Encounters of the Third Kind are Greys.
 * The Duros from Star Wars are also physically Grey-like, despite being considerably taller than most other depictions.
 * Ankh-Venharis from the 1982 film Time Walker is revealed to be a Grey-like alien.
 * In the Dilbert animated cartoon series, a one-off gag has a pair of Greys arguing when one refers to itself as a "UFO", defending its position by insisting that it's a better name than their real one, Zyzphlahelmrn.
 * The Velgroxs from Transformers: Rescue Bots, appear to be a hideous Grey-like species.
 * Giygas from the Mother game series somewhat resembles a Grey in Mother 1.
 * The Sophons and Mavros from Endless Space are Greys with Spacesuits.
 * The Arcturans' appearance in Goosebumps 2: Haunted Halloween is based off the Greys.
 * In Transformers: Cyberverse, one of the heads on Starscream's Quintesson Judge body resembles a Grey.
 * In Samurai Jack, one large muscular Grey-like alien warrior.
 * The Abductors from Star Trek: Untold Voyages #5 are typical Greys.
 * In Final Space, the Lord Commander and Clarence both somewhat resemble Greys.