They can be found in folklore from across the world and are an almost universal part of human society being nearly as old or older than humanity itself and seem to embody many of the fears and anxieties humans share as a species. Unlike demons, who are considered a spiritual evil, most monsters are considered fantastical beasts or exceptionally gruesome humanoids capable of being defeated or kept at bay.
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 mortals, deities or God Himself as punishment for some great misdeed; normally a taboo such as cannibalism or the murder of children.
Monsters have served many purposes in history but their main feature is arguably to enforce ethics and discipline such as 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. Example being 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 days of old.
Monsters also provided a means to explain phenomena, cultures and creatures that people had not truly understood and this was especially prominent when people started travelling to other lands and saw strange people and animals that shocked and perhaps even frightened them. This caused humans to become ager to share their exploits with people back home these bewildered travellers may of told amazing stories (some may also of deliberately exaggerated the stories, since storytelling has, and remains, a major source of entertainment to many).