A Hag is a supernatural being found in many mythologies and folklore - although many Hags are neutral rather than evil they have become heavily associated with witchcraft and evil, to the point the two are often interchangeable (much like how Ogres and Giants have become somewhat merged in modern times).
Traditionally, a Hag was seen as a spirit or goddess in the form of a grotesque and hideous old woman and often had power over the elements or magic - thus gained fear and respect in equal lengths by the superstitious, who would try to either appease the more benevolent of these spirits or ward off the more malicious.
Hags were likely inspired by pagan beliefs dating back almost as far as human history and probably embody part of the ancient concept of a "Goddess" - as Christianity spread, however, Hags would inevitably become more and more demonized (along with many of the "fairies"): giving rise to tales of grisly old demon-witches with a taste for human flesh and so on.
Famous Villainous Hags
Black Annis is the name given to a monstrous witch in English folklore that was said to haunt the countryside where she would hunt down her 2 favorite meals, lambs and the flesh of humans (especially children) - she was said to have iron-claws instead of finger nails and would be as bold as to reach into the windows of homes and snatch children away to be devoured.
Black Annis was not a human, as her face was said to be blue in coloration and highly deformed - this coupled with her fondness for human flesh is highly suggestive of her being a Hag, a type of spirit that is often confused with a witch but is in reality a type of ancient goddess or demon.
Black Annis was also said to have a horrible habit of flaying her victims, leaving their skins outside her cave to tan - this grisly sight may of served to warn others to stay away from her abode and she was commonly used as a bogeyman to frighten children into good behavior out of fear that should they misbehave they would be spirited away by the old hag and devoured.
Baba Yaga is a legendary Hag from Russian fairytales and folklore who varies from a cryptic aid, a troublesome obstacle or a downright malevolent villain depending on the story - she is always depicted as a frightening old crone however and is said to travel the woods either via the use of a flying mortar or by the use of her enchanted home: which is said to have several pairs of chicken feet holding it up and is capable of movement. Her teeth are made of iron.
Baba Yaga is also often said to enjoy kidnapping children and devouring them, a trait quite common for witches in folklore - where they were often associated with evil: although a witch in appearance however Baba Yaga is actually more of a spirit than a mortal, a kind of monster akin to the Bogeyman in Slavic tradition.
But in other versions, she is more sympathetic then evil and more like a friendly figure and want to help the protagonist.
Scota Bess was an evil being from Orkney folklore who was best described as a malevolent Hag - she was said to of once inhabited the area known as Stronsay and sat upon a rock formation known locally as the Mermaid's Chair casting wicked spells by which to summon storms.
Scota Bess was thus known (and feared) as one of Orkney's storm witches and embodies the unpredictable and dangerous nature of the open sea - somewhat ironically Scota Bess' ancient throne would later become associated with fortune telling and lost some of its former sinister status.