Xenomorph

The Xenomorphs are the main antagonists of both Aliens and Aliens vs Predator franchise, they are extraterrestrial, endoparasitoid species with multiple life cycles, possibly originating from the planet Proteus (also known as Xenomorph Prime). One of the deadliest of all known alien species, these creatures need a host organism in order to reproduce. The appearance of the Xenomorph varies depending on the facehuggers host. The human phenotype is generally around 7–8 feet, and roughly 136.0 to 181.4 kilograms in weight, with a long, muscular tail and large, curved, oblong head. The Queen of this species is generally twice as large (they can grow even larger, some even up to 100 feet tall, and stronger if given time) and possesses superior speed, strength and intelligence.

Story
There are no solid facts as to the origins of the Xenomorph species; instead, there are many theories which cannot be confirmed. They may be an artificially created species, or they may have evolved naturally on a planet very different than our own.

In the film "Prometheus," it's implied that the Xenomorphs were created as biological weapons by the Engineers. To be specific, the android, David (an android built to be curious and human-like), infected Charlie Holloway with a drop of the mutagen and Charlie then impregnated Elizabeth Shaw creating a large Facehugger-like life form (aka hectopod or trilobite). Elizabeth, in pain from the life form growing within her then removes the life form by entering the automated surgery table on board Meredith Vicker's exclusive ship. The life form later encounters an Engineer and seeds him much like a facehugger; thus, creating the Deacon which resembled the Xenomorphs.

It is heavily implied and revealed that in Alien: Covenant, David (Weyland's original Synthetic AI Cyborg) was the possible chief creator/developer of the first Xenomorph eggs that contain the "facehuggers". Utilizing the now emptied planetoid Elizabeth Shaw and David landed on, preceding Alien: Covenant, David was able to combine the limited number of creatures and organisms available with the "black-goo" (not to mention, unfortunately, Elizabeth Shaw's female reproductive system) to develop the first "face-hugger" eggs. David knew he needed Human hosts to eventually gestate the Xenomorphs to create his fantasy of creating something even better than what created himself. The Xenomorphs have become to be known as "the perfect organism" throughout the franchise. Thanks to Alien: Covenant, we now know that the Xenomorph is the end result of the creation cycle. --> Engineer --> Human --> AI Synthetic --> Xenomorph.

But, based on the Alien vs. Predator, the aliens are pre-existing and have been hunted by the Predator. It is possible that the Aliens were annihilated in Alien vs. Predator: Requiem, but were resurrected by David in the Alien movie just before the were forgotten forever.

The Xenomorph could be the result of genetic manipulation by Mala'kak (also known as the "Engineers" or "Space Jockeys") as a terraforming mechanism about ten million years ago, but apparently it went horribly wrong. As a result, over the years, the Xenomorph has become an entirely new species in the galaxy. This theory has been twisted slightly by Prometheus, when the Mala'kak brought containers filled with an unknown substance (it closely resembles Xenomorph secretions in hives). This substance mutated harmless organisms into somewhat hostile creatures. When humans "infected" with this substance mate with one another it produced a different subspecies of "hecto-facehuggers" that can grow incredibly large. As shown in the movie, this new subspecies, regardless of who it infected, Mala'kak or human, would result in the "Deacon" or "Proto-alien".

Another theory, commonly accepted, touched on in the expanded universe, is they were the alpha predator of their own ecosystem on a nightmarish and harsh planet (some assume it to be Xenomorph Prime, but there is no solid evidence). Without their native ecosystem to keep them in check, they have since infested their own kind, and developed into a separate dominant species. Other parts of the universe began to take notice in this interesting species and began them. The Mala'kak, the Yautja, Humans, and an unspecified highly technological humanoid species introduced in the comic Alien: Reaper began using them as means of sport, terraforming mechanism, as biological weapons, food delicacies, and scientific experiments.

Alien Vs Predator: Infestation
The Xenomorphs are the enemies of the game.

Halo 5: Xenomorph Invasion
The Xenomorphs are the main antagonists of the game.

Mortal Kombat: Earthrealm Warriors
Alien is one of the collab characters of the game.

Project X Zone 3: War of Infinite Worlds
The Xenomorphs appear as Enemy Units, while the Queen appears as a Boss Unit.

Project X Zone 3-D
The Xenomorphs appear as Enemy Units.

Battle X Royale
The Xenomorph is one of the playable characters of the game.

Dark Horse vs. Capcom
Six the Xenomorph appears as a playable character from Dark Horse side, his rival is Felicia from Darkstalkers.

Square-Enix vs. Capcom: When Worlds Collide
Alien is one of the characters from Square-Enix side, his rival is Predator Warrior.

DC vs. Sega: Chaotic Justice
Alien has a possibility of being one of the guest characters of the game.

Trivia

 * The Xenomorph's design is originally featured in H. R. Giger's book Necronomicon, named after the eponymous forbidden grimoire from the stories of H. P. Lovecraft.
 * The Tarkatan Xenomorph is a strain spawned with the Tarkata of Mortal Kombat. It is featured in Mortal Kombat X.
 * Within its franchise, the Xenomorph has been referred to as "Kane's son", a "creature", a "serpent", a "beast", a "dragon", a "monster", a "nasty", a "thing", a "bug", among others.
 * Besides merging with the Predator franchise, the Xenomorph species has been featured in a myriad of other science-fiction and superhero sagas. It has been fought by several DC Comics characters, including Batman, Superman and the Green Lanterns.
 * Xenomorphs make frequent appearances in the humorous comic strip Brewster Rockit: Space Guy!
 * They appeared in the comedy show Robot Chicken.
 * The creature appeared in an episode of the cartoon comedy Animaniacs called "Space Probed".
 * The Xenomorph Queen has also briefly appeared in an episode of the comedy series Family Guy.
 * In an episode of South Park, Cartman is seen watching aliens and rephares they mostly come out at night throughout the episode.
 * A small Xenomorph appears as a dog-equivalent on Planet 51 and is shown to urinate acid.
 * Xenomorphs have also been featured or mentioned in some Star Wars material, and while their canonical status is highly debated, this information implies that the species at some point managed to travel to, and colonize, at least one planet in a galaxy far, far away.
 * The Xenomorph's method of reproduction is rather reminiscent of that employed by the Ixtl, a species featured in the novel The Voyage of the Space Beagle. The first Alien film has even been accused of plagiarism regarding the book, although the producers denied any possible influence or connection.
 * Another possible inspiration for the Xenomorph is the small deep sea crustacean Phronima, which is physically similar to some Xenomorphs (particularly the Xenomorph Queen) and is also an endoparasitoid, laying its eggs inside the body of deep sea salps and killing them from the inside, after which the crustacean continues to inhabit the salp's hollowed body.
 * The Xenomorph's design has inspired the creation of the Metroid villain known as Ridley, which is a reference to Alien's director Ridley Scott. They also bear a resemblance to Space Pirates, the antagonist species in the Metroid game series.
 * In the Dragon Ball Z series, the first main villain Frieza has a third form transformation that resembles this species of alien life (the signature long cranium).
 * A Xenomorph named Heinrich serves as the final boss of the Nintendo 64 game Conker's Bad Fur Day.
 * Judging from the fact that virtually any Xenomorph can turn into a Praetorian and then a Queen, they are probably all female and reproduce asexually; which is called parthenogenesis, several organisms apply this reproductive ability. Either that or they are able to change sexes during life as is the case with some species of fish and amphibians. Or being hermaphroditic: capable of having both sexes.
 * In Aliens, the Xenomorphs seem to shake their head slightly before attacking their prey.
 * Some fans have noted similarities between Xenomorphs and the Propagators found on the Ragnarok in Final Fantasy VIII. It is possible that the Xenomorphs served as the inspiration for this creature, which is territorial, carnivorous, and is found only in remote locations in space.