Hag

# Background
A hag (or crone) is a wizened old woman, or a kind of fairy or goddess having the appearance of such a woman, often found in folklore and children's tales such as Hansel and Gretel.  Hags are often seen as malevolent, but may also be one of the chosen forms of shapeshifting deities, such as the Morrigan or Badbh, who are seen as neither wholly beneficent nor malevolent. The term appears in Middle English, and might be short for hgtesse, an Old English term for witch.

# Hag in folklore
A hag, or "the Old Hag", was a nightmare spirit in British and Anglophone North American folklore.  This variety of hag is essentially identical to the Anglo-Saxon mra  a being with roots in ancient Germanic superstition, and closely related to the Scandinavian mara. According to folklore, the Old Hag sat on a sleeper's chest and sent nightmares to him or her. When the subject awoke, he or she would be unable to breathe or even move for a short period of time. Currently this state is called sleep paralysis, but in the old belief the subject had been hagridden. It is still frequently discussed as if it were a para-normal state.
In Irish and Scottish mythology, the Cailleach is a hag goddess concerned with creation, harvest, the weather and sovereignty.  In partnership with the goddess Bride, she is a seasonal goddess, seen as ruling the winter months while Bride rules the summer.  In Scotland, a group of hags, known as The Cailleachan (The Storm Hags) are seen as personifications of the elemental powers of nature, especially in a destructive aspect.  They are said to be particularly active in raising the windstorms of spring, during the period known as A Chailleach.
Hags as sovereignty figures abound in Irish mythology.  The most common pattern is that the hag represents the barren land, who the hero of the tale must approach without fear, and come to love on her own terms. When the hero displays this courage, love, and acceptance of her hideous side, the sovereignty hag then reveals that she is also a young and beautiful goddess.
The Three Fates (particularly Atropos) are often depicted as hags.
In Persian folklore, the Bakhtak  has the same role as that of "the Old Hag" in British folklore. The Bakhtak sits on a sleeper's chest, awakening them and causing them to feel they are unable to breathe or even to move. Bakhtak also is used metaphorically to refer to "nightmare" in the modern Persian language.
Many stories about hags seem to have been used to frighten children into being good. Peg Powler, for example, was a river hag who lived in river trees and had skin the color of green pond scum.  Parents told their children that if they got too close to the water she would pull them in with her extra long arms, drown them, and sometimes eat them.  The parents hoped that the children would be afraid of the hag so they wouldn't go anywhere near the water.  That way, they'd never fall in and drown.  Peg Powler has other regional names, such as Jenny Greenteeth from Yorkshire and Nellie Longarms from several English counties.
Many tales about hags do not describe them sufficiently to make it clear whether the tale deals with an old woman who has learned magic or a supernatural being.

# In neurobiology
The expression Old Hag Attack refers to a hypnagogic state in which paralysis is present and, quite often, it is accompanied by terrifying hallucinations. When excessively recurrent, some consider them to be a disorder, however many populations treat them as simply part of their culture and mythological world-view, rather than any form of disease or pathology.

# In popular culture
In the Dungeons & Dragons game, "hags" are at least three races of female creatures, sort of female counterparts to ogres. They are the annis (named from an analogous creature from the British folklore), the green hag (a green-skinned version of the Slavic Baba Yaga), and the sea hag (sort of a sea witch, not a mermaid). All three sorts are evil, but not overly powerful.
Hags are occasionally mentioned in the Harry Potter series, but never in any great detail (the prologue of "Fantastic Beasts and Where to Find Them" mentions that they are classed as beings (as opposed to beasts) that children are part of their diet and that they can glide). Hags are occasionally encountered in the wizarding village Hogsmeade, where they are distinguished from "conventional" wizards and witches. It is unclear if such Hags live in Hogsmeade or simply visit the village for business and/or social reasons.
Hags are also mentioned in the Chronicles of Narnia. In The Lion, the Witch and the Wardrobe, Hags are one of the various kinds of evil creatures whom the White Witch has present at the killing of Aslan. Later, in Prince Caspian, a Hag, along with a Werewolf and the dwarf Nikabrik, tries to persuade Caspian to summon the Witch back to life. They attack after being refused, and are killed.
In the Popeye comics and cartoons, Popeye is sometimes pursued by a villainous witch called Sea Hag, who has an unrequited love for the sailor.