A monster is any creature, usually found in legends or horror fiction, that is often hideous and may produce fear or physical harm by its appearance or its actions. The word "monster" derives from Latin monstrum, meaning an aberrant occurrence, usually biological, that was taken as a sign that something was wrong within the natural order.

The word usually connotes something wrong or evil; a monster is generally morally objectionable, physically or psychologically hideous, or a freak of nature. It can also be applied figuratively to a person with similar characteristics like a greedy person or a person who does horrible things.

The root of monstrum is monere—which does not only mean to warn but also to instruct, and forms the basis of the modern English demonstrate. Thus, the monster is also a sign or instruction. This benign interpretation was proposed by Saint Augustine, who did not see the monster as inherently evil, but as part of the natural design of the world, a kind-of deliberate category error.

Well-known monsters in fiction include Count Dracula, Frankenstein's monster, werewolves, mummies, and zombies.

Monsters are a staple of fantasy fiction, horror fiction or science fiction (where the monsters are often extraterrestrial in nature).
Monsters are commonly encountered in fantasy or role-playing games and video games as enemies for players to fight against. They may include aliens, legendary creatures, extra-dimensional entities or mutated versions of regular animals.

Especially in role-playing games, "monster" is a catch-all term for hostile characters that are fought by the player. Sentient fictional races are usually not referred to as monsters. At other times, the term can carry a neutral connotation, where it is used to refer to fictional creatures that resemble real-world animals, or where "monster" is synonymous with "person". Characters in games may refer to all animals as "monsters".

Monsters? There is also talk of other fabulous human portents, which are not real, but invented: they are symbols of a set reality. This is the case of Geryon, king of Spain, of whom it is said he was born with three bodies: in reality, there were three brothers who got on so well that it was almost as if the three bodies shared one soul. This is also the case with the Gorgons, prostitutes with snakes for hair, who with one look turned men into stone. They were said to have only one eye, which they took turns in using. In reality, they were three sisters all equally beautiful, almost as one to the eye, the sight of whom stunned men so much they fancied that the sisters had turned them into stone.