Creatures form a set of diverse supernatural species in the universe of American Horror Story. Some creatures are depicted as humans that have been modified (either while living, or crafted from cadaver parts) through magic, science, or an unknown alchemical process with elements of each. Others are ancient and/or otherworldly beings, many of which hail from origins that are shrouded in dark mystery.
In "Murder House"
Ghosts are the souls, or spirits, of deceased people that can appear in a visible manifestation to the living. They appear as human beings, albeit ones who eternally remain the same physical age which they've died, oftentimes still adorned in the same clothes they were wearing at their time of death. They are tethered to the area of their death and can not normally leave it, though this restriction does not apply on Halloween, Devil's Night, or if they are released by magical means. Oftentimes, ghosts remain trapped in these locations due to negative occurrences that have happened in these locations' vicinities (i.e. rape, murder), or because they have unfinished business in life.
By their very definition, ghosts will exist forever and never die, as they are the spirits of those who are already deceased. Some ghosts remain trapped in an unbreakable chain for all of eternity, repeating themselves over and over again on an endless loop until they find their purpose, sometimes even unaware of their own deaths. This does not mean, however, that upon finding their purpose they will be able to leave the place of their death. On the contrary, after becoming aware of their new status, they are able to act on the physical plane, regaining memorization and cognitive abilities rather than being disoriented shells of the people they once were. Malevolent spirits, such as serial killers and mass murderers, tend to use their deceased status to their advantage, tormenting the lives of their fellow spirits or mortal victims without physical repercussions.
Although ghosts have no physical mass, they can consciously choose to become tangible or intangible at will, either manifesting in a corporeal form to interact with the living or take on an incorporeal form, during which no human can perceive or touch them and they are invisible. Ghosts do not follow the rules and laws of physics, and are able to move instantaneously from one location to another (within the confines of the place of their death) without physically occupying the space in between.
The Infantata is a bloodthirsty entity created by Dr. Charles Montgomery via the grafting of various animal parts to the remains of his murdered son, Thaddeus. It resembles an emaciated, humanoid figure with entirely black eyes, leech-like teeth, and long fingers topped by sharp, yellowed fingernails. Despite appearing elderly, it appears to be incapable of death, as it has been lurking in the basement of the Murder House for over nine decades. The phrase "Go Away" is oddly effective against it, as it will disappear similarly to a ghost.
According to psychic medium Billie Dean Howard, the Anti-Christ foretold in the Bible is the product of copulation between the living and the dead, a malevolent being that is fated to bring about the end of the world. Michael Langdon, conceived via the rape of human Vivien Harmon by ghost Tate Langdon, is believed to be the living embodiment of unholy evil, masking itself as a mortal despite its horrifying true form while in utero  .
Raspers are human beings horrifically altered by the experiments of Dr. Arthur Arden, a former Nazi scientist who amputated and injected patients with various diseases in an attempt to create humanoids that could survive nuclear fallout. Primarily culled from the inmates of Briarcliff Manor, they dwelled in the surrounding forests and subsisted on a carnivorous diet of raw organs.
A mysterious, otherworldly race of inhumans that hail from a different galaxy, possessed of advanced technology that far surpasses our own. They appear to their test subjects as tall, unnaturally slender aliens with slimy grey skin, four long fingers, deep-set brows above circular, ink-colored eyes, and undulating, vagina-like mouths, with tentacle-like traits similar to that of an octopus. Although their exact intentions and motivations remain unknown, they appear to have a significant interest in humanity and its sciences, abducting individuals in rays of light to conduct strange body alterations on them. They are responsible for the capture of Kit and Alma Walker, Grace Bertrand, and Pepper, each of whom they have modified in a different way as part of a larger unidentified agenda.
The Demon was a sly, ambitious, and malevolent fallen angel capable of possessing the bodies of unwilling individuals as its hosts, passing from one host to another when each no longer proved to be a sufficient vessel. At least while on Earth, it lacked a true form of its own, and thrived solely through the corruption of unwilling humans' bodies, slowly consuming their souls in order to take full ownership over them. It possessed many powers with which to carry out its vicious behavior, among them telekinesis and mind-reading, where it took great joy in seeing other's darkest secrets and manipulating them for its own gain and amusement. It had an intense hatred of God and humanity, and sought to take over the Catholic Church in order to distort the Christian faith and cast its followers into sin.
Unfortunately, because the Demon thrived solely through possession, it could not escape the kiss of Shachath, the Angel of Death, as failure to escape its previous host's body in time would result in it dying along with them, making the host both its strength and ultimate weakness. Given its status as an immortal being, however, it has presumably been cast back into Hell.
Shachath a.k.a. The Angel of Death
Shachath, also known as The Angel of Death, is a benevolent angel that appears to administer her kiss of death to individuals who are dying or wish for death. She takes the guise of a calm, soft-spoken elderly woman who arrives seemingly out of nowhere, adorned in black funeral attire consistent in style to the 1940s era. She appears only to those who are suffering or seek her presence, and is soundless and invisible to everyone except for her chosen witnesses. She shows great sympathy for the plights of those on the brink of death, asking if they are ready to pass on to the afterlife with sadness and compassion. Should her chosen subject state that they are not ready, she disappears without a trace. Should they state that they are, she kisses them softly on the lips, during which a pair of black wings sprout from her back. She then takes them to wherever they belong after their demise.
Witches are human beings that have been born with a unique genetic heredity that allows them to practice magic, or the power to affect change by supernatural or paranormal means, also known as "witchcraft". Witches are generally female, though there are also a few male witches, referred to as "warlocks". Because of their unnatural abilities and supposed connections with dark forces, witches have historically been feared, persecuted, and hunted. Many escaped the Salem Witch Trials and sought refuge in New Orleans, where they have since rivaled the native Voodoo practitioners. Among the population of witches, there is always one witch per generation who possesses a class of seven powers considered to be advanced acts of magic (referred to as The Seven Wonders), who holds a sacred duty to protect and lead her coven as Supreme.
Voodooist Witches are practitioners of Voodoo, a magical-religious practice of West African origin. Unlike Salem descendants, humans can become Voodooists through practice and training. In the 16th century, Voodoo spread throughout the Caribbean and United States by the slave trade, and became particularly relevant in Louisiana. According to Marie Laveau, Voodoo is far older than witchcraft, hailing from shamanistic practices thousands of years old. She claims that the slave girl Tituba is responsible for granting magic to the girls of Salem, thereby creating the first witches, though it is unknown if these claims are true. The status of Voodoo Queen is equivalent to the status of Supreme.
Voodooists believe in entities called "Loas", whom they worship through rituals synchronized with Catholic saints. The only Loa that has appeared thus far is Papa Legba, the Gatekeeper of the Spirit World.
Bastien, a house slave of Delphine LaLaurie, was tortured and mutilated by her after being caught with her daughter, who forced herself on him. This culminated in Delphine's execution of him by placing a severed bull's head over his own, as she rejoiced in having made her own Minotaur.  He was reanimated by his lover, Marie Laveau, with Voodoo magic, at the cost of losing what remained of his humanity.  Over time, the bull's head has integrated itself to his body and he developed additional bull-like characteristics, having changed fully from man to beast. 
Corpses that have been reanimated by magic. They are merely puppets of the ritual caster, such as those raised by Marie Laveau to exact vengeance on her enemies. The end result seems largely based on the intent of the caster and the rite used.
Because they are rotting corpses, they are immune to the magical power of injury transference, as they have no working nerve-endings or consciousness of their wounds.
Note that this is different from those who have been restored to life by the resurgence ability. Those affected by resurgence are no longer dead and are alive, not undead.
Immortals are humans that have been fortified to be unaging, and eternally require no sustenance. They are not immune to injury, but their individual body parts will continue to survive once severed. Marie Laveau gained immortality by making a pact with the Voodoo god Papa Legba, which entailed her having to sacrifice an innocent soul every year at an allotted time.  Delphine LaLaurie was cursed with immortality when she drank a vial of Marie Laveau's tears - a punishment enacted by the Voodoo Queen for her torture of black slaves, including her lover, Bastien.
In "Freak Show"
The carnies have a superstition that any performance on Halloween, even trivial ones, risk summoning Mordrake. Once he appears, an ominous thunderstorm brews on the night horizon, casting streaks of unnatural lightning in the sky, while a thick, bright green fog swirls wherever he treads. The whisperings of his second face can be vaguely heard telling him what it desires: the life of one of the carnies.
The Afflicted, also known as Vampires, are humans who have been infected with an ancient blood virus that grants eternal youth and life, among other supernatural characteristics, at the cost of requiring a diet of fresh human blood.
They seem to look exactly like regular humans, oftentimes known for being "beautiful" due to one attractive afflicted spreading the virus to another attractive person, though they can also appear average. Upon being turned, a human inflicted with the virus dies before returning to life as one of the afflicted, stripped of their mortality; a person who is already dead can also be brought back to life via the virus's effects. As a result, the afflicted have very pale skin and low body temperature, at 75.5 degrees.
They cannot age or grow old since they stop aging once turned; Ramona Royale compares this to being "frozen in amber". The virus causes biological changes in the hosts, endowing them with heightened senses and greatly increased physical abilities such as a supercharged immune system, granting them immunity from illnesses, diseases, viruses and other infections. They also possess a variety of superhuman physical abilities, including enhanced strength, speed, agility, reflexes, senses, endurance, durability and accelerated healing, as well as the power hold sway over their chosen victims' minds, enthralling them in order to feed on their blood. They are able to eat normal food as well, though it does not sustain their life or provide nutrition in the same way as blood.
They have increased sensitivity to light, especially sunlight, which momentarily weakens their supernatural qualities. They cannot appear in photographs, as still images of them appear blurry and unidentifiable, mistaken for technological problem by the unwary. They don't die from starvation and suffocation, but can be killed via severe physical trauma or firearms, due to hemorrhages and bleeding. Their blood virus cannot heal mental illness, such as dementia or Alzheimer's, and merely halts these illnesses from progressing opposed to healing the neurons.
NOTE: The term "afflicted" is used as a placeholder here until a term is given inside the show's universe.
The Addiction Demon
A gaunt being with pale, withered skin similar in composition to the dripping wax of a candle, with long-fingered hands ending in sharp black talons. It lacks any distinguishable facial features, and has only the stump of where a nose should be on its cadaverous, melted-looking face, with no eyes or mouth visible. It is extremely sexually violent in nature, and feeds off of the pain and fear of its rape victims, assaulting them with a strapped on conical drill-bit dildo. Its presence is heralded by the strange flickering of lights in its vicinity, and it can stare out at individuals from behind the glass of mirrors or other reflective surfaces. According to James Patrick March, it was supposedly brought into creation by Sally and various other drug addicts that used the Hotel Cortez to shoot up drugs, though the means through which this occurred is unknown.
Scáthach is a savage immortal witch who practices ancient dark magic and worships the old gods. She has the appearance of a filthy crone encrusted with dirt and blood, possessed of sharp yellow fangs, mossy greenish skin pervaded with black veins, long scraggly russet hair topped with an animal skull (which she uses as a ritualistic headdress), and unnatural, mismatched eyes. She screeches like a vulture, and is able to manipulate human beings into selling their souls to her in eternal servitude. According to Ryan Murphy, Scáthach is the original Supreme, pre-dating the Salem witches by centuries. 
The Old Gods are a pantheon of dark and bloodthirsty deities which belong to ancient beliefs predating Christianity. They possess great and terrible powers, and imbue their followers with dark magic and immortality. In exchange for the blood sacrifices of humans and animals, the old gods grant or enhance the fertility of the earth, turning barren plains into fertile fields, and encourage orchards to bare fruit and flowers to bloom, bringing good harvest and mild weather. The Dying Grass Moon in the month of October, also known as the Blood Moon, can be considered both a corporeal and symbolic representation of the old gods' presence.
- ↑ 1.0 1.1 Episode: Halloween: Part 1
- ↑ Episode: Devil's Night
- ↑ 3.0 3.1 Episode: The Axeman Cometh
- ↑ Episode: Room 33
- ↑ Episode: Piggy Piggy
- ↑ Episode: Bitchcraft
- ↑ Episode: The Replacements
- ↑ Episode: Fearful Pranks Ensue
- ↑ Episode: Burn, Witch. Burn!
- ↑ Episode: Head
- ↑ Episode: The Magical Delights of Stevie Nicks
- ↑ Episode: Boy Parts
- ↑ Ryan Murphy Confirms Lady Gaga’s Character Is The Original ‘American Horror Story’ Supreme.