A monstrum is a sign or portent that disrupts the natural order as evidence of divine displeasure.[327] The word monstrum is usually assumed to derive, as Cicero says, from the verb monstro, “show” (compare English “demonstrate”), but according to Varro it comes from moneo, “warn.”[328] Because a sign must be startling or deviant to have an impact, monstrum came to mean “unnatural event”[329] or “a malfunctioning of nature.”[330] Suetonius said that “a monstrum is contrary to nature <or exceeds the nature> we are familiar with, like a snake with feet or a bird with four wings.”[331] The Greek equivalent was teras.[332] The English word “monster” derived from the negative sense of the word. Compare miraculum, ostentum, portentum, and prodigium.

In one of the most famous uses of the word in Latin literature, the Augustan poet Horace calls Cleopatra a fatale monstrum, something deadly and outside normal human bounds.[333] Cicero calls Catiline monstrum atque prodigium[334] and uses the phrase several times to insult various objects of his attacks as depraved and beyond the human pale. For Seneca, the monstrum is, like tragedy, “a visual and horrific revelation of the truth.”[335]

Source: Glossary of ancient Roman religion – Wikipedia