Monsters are frightening creatures that can be found in folklore from across the world and are an almost universal part of human society, being nearly as old as humanity itself and seeming to embody many of the fears and anxieties humans share as a species. Unlike demons (which are considered a spiritual evil), most monsters are considered to be fantastical beasts or exceptionally gruesome humanoids capable of being defeated or kept at bay.
In many legends, a hero is born who vanquishes a particularly dangerous and hostile monster, and earns themselves a great reward, though other tales see the hero suffer as a result yet even in death a hero who slays a fearsome monster is held in high regard by their respective society and often become lasting symbols of hope to those who believe in the legends.
Often in folklore a monster is the forbidden offspring of some blasphemous union (such as human and animal or (in some cultures) human and divine (especially angels) ). They may also be the result of black magic or a curse given by either mortal, deities or God Himself as punishment for some great misdeed (normally a taboo such as cannibalism or the infanticide).
Monsters have served many purposes in history but their main feature is arguably to enforce ethics and discipline, warning of the dangers of engaging in unlawful or "sinful" ways and also being an easy way to keep young or superstitious people from endangering themselves (such as telling tales about evil water-hags or bugbears in the forest so as to discourage dangerous play near rivers or venturing too far into forests, which may of been populated by wild animals and criminals in the old days).
Monsters also provided a means to explain phenomena, cultures, and creatures that people had not truly understood. This was especially prominent when people started travelling to other lands and saw strange people and animals that shocked and perhaps even frightened them. Eager to share their exploits with people back home these bewildered travelers may have told amazing stories (some may also have deliberately exaggerated the stories, since storytelling has (and remains) a major source of entertainment to many).