A monster is any fictional creature, usually found in legend

Legend

A legend is a narrative of human actions that are perceived both by teller and listeners to take place within human history and to possess certain qualities that give the tale verisimilitude...

s or horror fiction

Horror fiction

Horror fiction also Horror fantasy is a philosophy of literature, which is intended to, or has the capacity to frighten its readers, inducing feelings of horror and terror. It creates an eerie atmosphere. Horror can be either supernatural or non-supernatural...

, 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

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 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.