Demon

A Demon, Pterodactyl (Russian: Птеродактиль), or "Vichukha"; regionally known as gargoyles in the Volga and batwings in Vladivostok; as referred to in several Universe novels, is a species of large aerial surface predator that appears throughout the Metro Series. One of the only known flying mutants in post-apocalyptic Moscow, they are at the top of their food chain and deadly to both humans and mutants alike.

Overview
A demon can be described as having a knobbly spine, short tail, and three pairs of limbs: two prehensile arms, two legs, and two large wings. They also have a long head with a blunt snout, consisting of a bat-like nose, eyes with vertical cat-like slit pupils set far apart on the head, and a large mouth with 4 canines and several secondary teeth. The backs of demons are covered in sparse, short black fur and there are folds of skin on the base of the neck. Their wings are attached to their body with a massively muscled arm-like limbs that are almost twice the diameter of their thigh muscles. This explains how demons can fly, despite their massive size - a demon's wings are in fact so powerful that they can hover in the air for significant periods of time. Demons vary in size from babies the size of household cats to the largest encountered in the level, "The Crossing"; this demon is the size of a large horse. Demons produce roars that seem to be a guttural version of a tiger's growl. Because their eyes are quite small and on the sides of their head, it's unlikely that the demons hunt by sight. Their large nasal ducts suggest a powerful sense of smell.

Genetic Origin
It's unknown what exactly demons mutated from and there are a number of theories suggesting mammalian, avian, or reptilian origin. However, none have so far been proven.

The most likely animal they have mutated from was the bat. Demons possess hair while lacking feathers, scales, or beaks, suggesting a mammalian origin. They also have very bat-like noses, seem to fly like bats, and sometimes swoop down like bats upon their prey. Another aspect that would support them being descendants of bats would be that in order to achieve such a drastic form of mutation in such a short time, they would have to have a fairly high birth rate. This theory is supported by the fact that most of the other mutants seem to be descended from fast breeders too. Nosalises are presumed to have been descended from moles or shrews, or like one person in theatre suggests elephants from the zoo, while lurkers and watchmen possibly derive from rodents.

Further evidence from Metro Exodus validates the theory of chiroptera - bat origins for the demons; as their appearance and anatomy, such as elongated ears (used by bats for echolocation) and elongated teeth (similar to vampire bats and fisher bats), greatly resembles pre-war bats. At least one demon can also be seen resting by hanging upside-down; exactly like a bat.

Opposing the bat theory is the fact that their claws and hind legs are similar to feline or canine hind legs.

Unlike all other mutants and every other vertebrate, demons have six limbs. While this may simply be an extensive mutation that was quickly brought into their gene pool following the nuclear war, it's also theoretically possible that, given the many different physical characteristics that are drawn from several different unrelated species, that demons are some kind of genetically-engineered chimaera-like animal that was created in a lab before or not long after the war. This is not an entirely unlikely prospect, given that Khan raises the possibility of the Dark Ones also being genetically-engineered.

Their roars strongly resemble those of a lion, tiger, or other large cats (as opposed to the screeching of a bat), and they possess vertical slit pupils similar to felines. These features, in addition to the layout of their teeth, would seem to support the possibility that the demons are descended from tigers, which were native to some parts of Russia before the war. However, the relatively slow birthrate of tigers would seem to discredit this. This could mean that they mutated from ordinary house cats, which were more numerous and bred faster than tigers. At times, they do display some distinctly feline-like behaviour. The baby demon in Metro: Last Light reacts to Artyom's presence by arching its back and hissing like an agitated house cat, while an adult one encountered on the bridge paws at the cable that Artyom rides along before biting it. There exists evidence that debunks both feline and bat origin theories. In Metro: Last Light, Demons possess a relatively similar size, shape, and behaviour but have undergone significant external appearance changes. They have a distinctly reptilian appearance with scaly skin and have lost their fur or hair; contradicting both the feline and the bat origin theories. Demons also appear to have become even more muscular than they were before. Their talons are now a bit shorter, but are thicker and wider. On top of all this, they have a significantly different head shape, their teeth are in different places and more uniform in size and shape, and their jaws and eyes are in slightly different places and positions. Demons never have projecting ears, instead possessing ear-holes similar to that of reptiles, birds, and amphibians. This might indicate the possibility of demons being reptilian or amphibious in origin.

A third theory states that the demons may be the results of genetic experiments; which are known to exist as the biomass and giant amoeba. The species being true chimeras would easily explain their highly differentiated behaviour and anatomical patterns; such as possessing 6 limbs; an evolutionary trait that only insect species ever developed on Earth.

Habitat
Demons are purely surface-dwelling predators due to the fact that they cannot enter the metro due to their large size. Demons often make nests on top of tall buildings, giving them a good vantage point to hunt for prey and to offer protection for their young. They also will try to find areas with large open grounds, such as parks, where hunting is easier. They hunt either alone or in small packs, spread out across several city blocks. If one is seen, there is likely to be more close by. While they prefer to attack by circling overhead and swooping down on prey to wound it or carry it away, if they can catch humans in the open they will sometimes land to attack. They tend to avoid entering structures as this limits their ability to escape by flying if the situation turns against them, but in some occasions, they will attack through windows, doors, or damaged walls to get at prey inside. However, on at least one occasion a demon entered a hole in the roof of the large chamber in the library and was deft enough to launch swooping attacks without running into any walls, occasionally hovering in place. They are not easily intimidated and will readily attack creatures much larger than themselves, such as a Bog Shrimp. One particular demon, the Seraph, has developed a symbiotic relationship with a smaller species of mutated bats, utilizing a swarm of them to stun its prey, as well as camouflage itself in the sky, often giving the impression that the beast is appearing from thin air. How exactly the demon is able to control this swarm is unknown, but this ability, combined with its large size and strength, made it the apex predator of Vladivostok.

Diet
Demons will hunt almost anything they see but usually prefer smaller prey that can be carried in its talons, either to drop and kill it or to drag it back to the Demon's nest. They hunt nosalis, watchmen, amphibians of all kinds and humans. They have also been seen attacking bog shrimp, though it's unknown whether this was for food or territorial disputes.

Tactics
In Metro 2033 and Metro: Last Light, demons are often seen flying through the irradiated skies of Moscow, though it is rare that you will have to fight one. However, if Artyom finds himself forced to do, keep in mind that they are a formidable foe and can easily kill him, even on easy difficulties. It will take quite a few shots, between six and twelve, from a Shambler to kill it and almost three magazines from a Kalash. A sticky grenade can often kill one instantly. However, it will be almost impossible to stick one to it in the air, so they must land first. On lower difficulties, one revolver cylinder (six rounds) can suffice.

Demons prefer to perch in only a handful of locations. It is possible to locate an area of cover, with a view of one of these positions, and inflict head-shots on the demons with a scoped weapon. They will usually take off after the first few shots, but eventually return to the same position. A scoped revolver is ideal for this method of attack, being able to kill the demon with relatively few shots. Demons are able to land in places other than their preset perches, however, and may attempt to do so if Artyom is hiding in a spot with a lot of horizontal covers but no roof, such as on the top floor of the Nazi outpost.

Fighting demons in the open is an extremely dangerous endeavour; if Artyom insists on doing so, his only hope is to make absolutely sure that there is a tall, solid object behind him, preventing them from making swooping attacks, forcing them to land where they are a larger target. However, the best way to deal with a demon is to simply run - if Artyom darts from building to building, they will be unable to reach him and will not be a threat. This is not useful when in open territory, but they can still be safely avoided if Artyom moves swiftly.

In Metro Exodus, demons will fly around large structures only, but they will also tend to fly around in some areas where they could hunt prey, as a result, pose a potential threat to Artyom when they fly too low. As such, the best course of action is to run to cover to avoid getting dragged by one, like the game's predecessors. However, as dangerous as they can be, they are actually less difficult to kill, only taking 2-3 shots with a revolver to eliminate one, but they are difficult to hit with, as they are mobile enough while roaming around the air (except for one located in a silo in the Volga, where it dwells within a nest), thus Artyom should leave them be while wary of the skies.

In the Novels
In Metro 2033, demons are the first surface creatures seen by Artyom on his trip to the library from Polis, where one is seen nesting on a church spire. They are described as having leathery wings and are often called "pterodactyls" by other stalkers. However, unlike in the game, they are described as having no neck, with long heads jutting from their chest. Demons are encountered again when Artyom is pursued by watchers when one swoops down and carries off a watcher to eat. Later, a demon attacks Artyom outside VDNKh station, as it has made its nest directly above the station entrance. It also attacks the stalker vehicle, a scene used again in the video game when demons pull over the vehicle in the Prologue to Metro 2033.

They are also seen in Metro 2034, by Sasha when she runs away to the surface. Whilst exploring the city, two demons spot Sasha and attempt to carry her off. While she dodges the attack and shelters near a wall, the demons work out her plan by landing. She is only saved by the timely arrival of a stalker from Hansa who uses an explosive to obliterate one demon and scare off the other.

Metro 2033 (the book)

 * Some of the stalkers tell Artyom that the demons might actually come from the native birds which resided in Moscow before the blast.
 * It's mentioned that some demons made their nest right on the top of the Cathedral of Christ the Saviour.

Metro 2033 (the game)

 * Bourbon jokingly prefers to call them "bitches", as he says in the mission "Dead City 1".
 * As demons prefer high locations from where they can spot their prey, the largest amount of demons encountered in-game can be found at Ostankino Tower, the highest point in Moscow. Roughly six are seen at once, descending from the tower to attack the Rangers as they are busy fighting watchmen.
 * None of the demons have to be killed and all can be avoided, except the one on top of Ostankino Tower. On the other hand, all demons can be killed (excluding the swarm from the Tower level) but can be particularly difficult to kill, as they only appear for a short period of time.
 * If Artyom gets pounced by a demon or picked up off the ground, he cannot survive. This often ends in death during levels such as "Outpost" and "Alley".
 * On "Dead City 2", Artyom has roughly 10–20 seconds to run from, or kill, the demon that attacks him and Bourbon, or it will pounce Artyom. This gives him just enough time to clear any watchers and run for cover.
 * During the level "Top", Artyom must kill the demon before it finishes off the injured Miller, or the mission will fail.
 * On "Alley", it's best to find the hole and get in as soon as possible. This is much safer than to stay and fight, as in this level the demon is highly likely to pounce on Artyom, and indeed unkillable, as it's required for the plot of the mission to continue.

Metro: Last Light

 * In "The Dead City" chapter, a demon nest is found with a baby demon inside. The Dark One will chastise Artyom for killing its mother and the baby. Also, he will gain a negative moral point. However, the Dark One will never inform Artyom or let him know about this ahead of time, only after the deed has been done.
 * This is also the best location for the Van Helsing achievement/trophy, as the demon doesn't stray too far from its nest.
 * In Redux, the baby demon can pick up Artyom. This is most likely a glitch.
 * Demons are known as Winged Ones by the Dark Ones, most likely as they have not heard the human name for them.
 * Most demons can be avoided and none need to be killed - avoiding doing so will sometimes award positive moral points.
 * The demons in Metro: Last Light have a distinctly more reptilian appearance and in fact, look very similar to deathclaws from the Fallout series with wings and fewer digits on their limbs.
 * Shrimp are speculated to eat Demons and vice versa.

Metro Exodus

 * The people on the Volga call them gargoyles.
 * The city of Vladivostok is home to a subspecies of demon called "batwings" by the residents. Roughly twice the size of a regular demon, and perpetually shrouded in swarms of regular-sized bats, they are also significantly rarer than regular Demons. Indeed, only one batwing is known to exist, dubbed "Seraph" by the local Bandits, and it's killed by Sam during his travel through the ruined city.
 * The term "demon" could be a moniker for any large, flying mutant with bat-like wings; albeit unlikely as the main flying predators are quite easily differentiated from the rest of the known flying mutant species.
 * Upon close inspection, demons in Metro Exodus appear to be male.