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.
The sign prophesied the approach of a calamity or misfortune, of public nature or to the nation as a whole, rather than to an individual.
Such a foreboding omen or portent was variously called. The term most frequently used was prodigium, though this might also denote, rarely, a favorable portent.
This, when borrowed into English use in the form prodigy, continued at first to mean something of extraordinary nature taken as an omen, but is now applied especially to a person or animal above the ordinary in some skill or talent or mentality.
Another term was 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, point out, familiar to us in the derivative, demonstrate.
But 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 unusual and frightening form, that had been taken as an omen of evil.
Thus, even among the Romans, monstrum, which became monster in English, was used also for anything abnormally large, or of unusual or frightening appearance.