A monster is any fictional creature, usually found in legends or horror fiction, that is somewhat hideous and may produce physical harm or mental fear by either its appearance or its actions. The word "monster" derives from Latin monstrum, an aberrant occurrence, usually biological, that was taken as a sign that something was wrong within the natural order.
The word connotes something wrong or evil; a monster is generally morally objectionable, physically or psychologically hideous, and/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.