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 and/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, 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.
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.