1. Any large, ugly, terrifying animal or person found in mythology or created by the imagination, especially something fierce that kills people. 2. In medicine, a fetus or infant with such pronounced developmental anomalies as to be grotesque and usually nonviable. 3. Etymology: from Latin, an omen; a supernatural manifestation; hence, "horrific-supernatural being; a supernatural manifestation".

Another perspective regarding monster

Any sign thought to have been given by the old roman gods, that is, any strange incident or wonderful appearance, was taken as a warning, a belief that the gods were provoked or upset.

The sign prophesied the approach of a calamity or misfortune, of public a nature, or to the nation as a whole; rather than to an individual.

Such a foreboding omen, or portent, was called a monstrum; which always, from the nature of the strange incident, denoted the approach of some catastrophe.

The term was derived from the verb monstro, "to show",or "to point out"; familiar to us in the derivative, demonstrate. Through the dread inspired by the term monstrum, that word also came to be applied to whatever was the "fearful thing", the "strange appearance of an unusual and frightening form", which was considered to be an "omen of evil".

Even among the Romans, monstrum, which became monster in English, was used also for anything abnormally large, or of unusual or frightening appearance.