Monsters are often featured in folklore and fairy tales as evil creatures resembling a mixture of different animals.
Primarily used in medical references to a fetus or an infant with substantial physical abnormalities considered by some to be like a "monster".
"Regardless of the form a monster takes, all monsters have one thing in common: they are all awe-inspiring, even frightening, by virtue of some aspect of their physicality or personality.
In fact, the word monster comes from the Latin monstrum, meaning "an evil omen" which in turn seems to be a derivative of the verb monere, "to warn, to remind".
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.