Xenomorph

Xenomorph XX121, commonly referred to as simply the Xenomorph, known colloquially as the Alien, and classified as XX121, is a highly aggressive endoparasitoid extraterrestrial species.
The Xenomorphs are vicious predatory creatures with no higher goals than the propagation of their species and the destruction of any life that could pose a threat to them. Like wasps or bees, Xenomorphs are eusocial, with a fertile Queen breeding a host of subordinate castes. The creatures are known for their potent acidic blood and their pharyngeal jaws, although their biological life cycle, in which their offspring are implanted inside living hosts before erupting violently from their chests, is in many ways their signature aspect.
Aside from their repeated conflicts with humans, Xenomorphs have notably been associated with the Yautja, who know them as kainde amedha ("hard meat"). The Yautja revere the creatures as incredibly dangerous and thus highly worthy prey, notably employing them in initiation Hunts that serve to test young members of their society and mark their progression into adulthood.

Biology

The physical form of a Xenomorph can vary widely depending on its life stage, hive, caste, age, and host. Its ability to incorporate genetic material from its host, coupled with the existence of numerous mutations and deviations which have originated naturally or artificially; makes the Xenomorph one of the most phenotypically diverse organisms in the galaxy.
A typical Xenomorph begins its life cycle as an egg-like creature (known as an ovomorph or simply egg) which, when detecting the presence of a potential host, opens up to release the second stage: a small arachnid-like life form (named the facehugger) which launches itself onto the host's face and impregnates it with an endoparasitoid larva. This parasite then consumes parts of the host from within and violently emerges from the host's chest cavity as a worm-like creature (grotesquely christened as the chestburster), which will quickly scurry away to find a safe place where it can molt and complete its metamorphosis into an adult Xenomorph, which happens, surprisingly enough, over the course of mere hours.
Adult Xenomorphs all tend to share some common traits: a vertebrate-like body, a protective dark exoskeleton, a long flexible tail, an elongated domed head and a toothed mouth hiding a secondary, extendable, proboscis-like inner maw. Little is known about their biology, but it is known that they eat with their inner or secondary mouths, not their outer mouths. This mouth is somewhat similar to the pharyngeal jaw of a moray eel. They also bleed an unknown and extremely corrosive fluid. This substance is dull yellow in color and often referred to as "blood", although whether it's used to transport nutrients through the body and/or serves principally as a defensive mechanism is not clear.
While the extremely corrosive fluids and tremendous resilience displayed by the creature might suggest an exotic kind of biochemistry, the fact that the Xenomorph not only feeds on Humans, but actually develops inside them, strongly suggests that its biology is carbon-based. The ability to inherit traits from its hosts also suggests that it incorporates genetic material from them, implying that the Xenomorph either has DNA, or is able to read and convert DNA into whatever means it uses to store genetic information. There is evidence that the genetic exchange is mutual (i.e. the parasite alters the host's DNA as well), given that Ellen Ripley's clones were created in an attempt to gain access to the Xenomorph Queen's genetic material, which wouldn't be possible otherwise.
Xenomorphs operate in hives made of a resin they produce, mixed up with their saliva fluids. The Queen can be generated in more than one way: from specialized royal facehuggers, or maturing from Praetorians, which in turn can mature from Drones or Warriors; by a process referred as 'molting'. Judging from the condition that any Xenomorphs can potentially 'molt' and grow into a Praetorian and then a Queen, the Xenomorph species is likely all unisexual and reproduces asexually by a process called parthenogenesis: a reproductive method practiced by many species on Earth and other planets. Alternatively, it's also possible that the aliens are either hermaphrodites or practice sequential hermaphroditism, i.e. being able to change sexes during life, as is the case with some species of fish and amphibians.

Intelligence

Most Xenomorphs appear to possess an intelligence level equivalent to that of a typical primate. It's well within their capabilities to learn relatively quickly, gathering information and making advantageous use of their environment. Although they do not demonstrate Human and Yautja-level intelligence as a species (such as abstract reasoning, introspection and technological advancement), their Queen appears to possess considerable acumen in her social behavior and manipulation of Human technology, such as cutting the power of the LV-426 colony and operating an elevator. While it may take considerable time for them to learn how to operate machinery, other Xenomorphs have displayed fast reasoning and the capacity to find creative, counter-intuitive solutions to problems, as exemplified by a group which, after noticing that they were being contained, intentionally murdered the weakest so that the acid of the Xenomorph's blood would eat through the floors.
As the adult Xenomorphs are known to inherit physical characteristics from their hosts (for example, when a Yautja was infected, the parasite later developed into a "Predamorph" or "Predalien") it is possible that the intelligence level of the creature may be influenced by its host as well. Furthermore, being an eusocial species, it is possible that the intellectual capacity varies for each different caste.
Xenomorphs have demonstrated little emotion, though they are not completely devoid of fear, such as when they are in the presence of fire, which they are vulnerable to. They have also been shown to communicate pain through shrieks or screams when attacked.
It has been suggested that Xenomorph nests may operate under a hive mind, or a collective consciousness directed by the Queen, who can call back and give orders to her subjects, both telepathically and vocally with shrieks. In some cases, the Queen Mother Xenomorph communicates telepathically with potential hosts, inducing terrifying dreams and religious visions, which may even lead to the formation of cults with the Xenomorph as a god-like figure, not dissimilar to the cults of Cthulhu and other such entities. The most famous example of this phenomenon might have been Walter Golic, who received supposedly telepathic messages from a captured Xenomorph; which ultimately led him to release it and let it kill him. He called this Xenomorph "the Dragon", and viewed it as a god.
There is evidence that the Xenomorph hive mind also includes a collective memory that passes along even at a genetic level. Through the tests conducted on Ellen Ripley, it is revealed that the memory passed from Ripley onto the new generation of Xenomorphs allows them to read and understand different languages. Events on the LV-426 colony and the USM Auriga show that the species excels at observational learning. Another case of observational learning can be seen following a Xenomorph escaping its cell. It had learned that the consequences of attacking the observatory glass was a jet of extreme cold (possibly liquid nitrogen), triggered by a large red button on the console. When a crewman entered the now-empty cell to study the hole through which the Xenomorphs escaped, one of them entered the control room and activated the button using its inner jaw.
In the rare event that a Xenomorph finds itself under attack or otherwise vulnerable, it might display incredible creativity and cleverness. They are capable of moving quickly and silently in ventilation systems, despite their size, and will use such conduits for ambush, escape, or simply as an easy way to travel between areas.
Xenomorphs are conscious of the effects of their acidic blood, and will use it to their advantage - to break out of Human-constructed confinement, or as a weapon. They show some military tactics (such as retreating when overwhelmed), and can find ways to get around certain problems (such as pushing the red button to freeze a soldier in liquid nitrogen in the afore-mentioned incident). They know that the darkness is their ally, and break lights to create cover.

Anatomy

A Xenomorph is equipped to be the perfect killing machine. They possess an exoskeleton much like that of a lobster, impenetrable to everything but the most powerful Human firearms (12-gauge shotguns, Pulse Rifles, and stanchion launchers are quite effective). Ash claims the creature's skin is made from "protein polysaccharides", so this would imply a substance similar to chitin. He also says the creature replaces its surface cells with polarized silicon, which is a very inert, heat resistant, and flexible polymer. This allows these creatures great environmental resistance. However, they are vulnerable to sudden, massive changes in temperature. It's been demonstrated that they can be destroyed by immersion in molten lead followed by sudden cooling with fire sprinklers, causing their silicon shells to shatter.
Attached at the ends of both of their arms are a set of long fingers and shiny, black, extremely sharp claws capable of slashing through almost anything within their grasp. Their tail is long and flexible. Some, though not all, individuals also have ridges on top, and a serrated toxic tip at the end of the tail to lacerate enemies.
They stand at around 7 feet tall (~ 2.13 m), averaging in anywhere between 140 and 180 kg; although size may vary wildly depending on the host chosen, as seen for instance with the "Jock-Xenomorph": a gigantic creature spawned from a Mala'kak host.
A Xenomorph's outer mouth is filled with sharp teeth, but their main killing device is the inner jaw that shoots out to penetrate directly into the victim's brain. The inner jaw (also known as an "attack tongue") is a muscular appendage on the inside of the Xenomorph's mouth. All Xenomorphs have this trait, excluding the genetically-engineered Newborn Xenomorph. It consists of a cord of muscle with a small but extremely deadly mouth-like appendage on the end. This attack tongue is able to punch through almost anything, from bone to Yautja armor.
The plasticity of the Xenomorph phenotype is one of the species' most notable features, and has resulted in physical traits that diverge from individual to individual and from hive to hive. As an example of how a host can influence the morphology and skills of the parasitoid, a Xenomorph spawned by a dog displays a distinctly canine body structure, digitigrade legs, quadrupedal movement, and greatly enhanced speed compared to those spawned by Human hosts, which are typically plantigrade and often bipedal.
Even among those spawned exclusively from Humans, different Xenomorph hives can produce highly divergent individuals. Some are taller and lankier, with an almost erect posture. Others are shorter and bulkier, and move more quadrupedally. While most have smooth craniums, others have ridges, which may be attributed to their age (i.e. those which have more time to develop may end up with ridges) and/or caste (ridges being associated with Warriors, and smooth features with Drones).
Some Xenomorphs have tails that are roughly the same length as the rest of their bodies, equipped with a small, almost surgical stinger-like barb on the end. This stinger can be, and has been, used to perforate victims, and may turn loose from the tail and remain lodged in the victim's body, not unlike a bee stinger. Conversely, other specimens have tails that are considerably longer, and armed with a large, knife-like blade at the tip and sometimes with ridges of spikes right before the blade, which may aid in swimming.