From Latin 'monstrum' (divine omen), from 'monere' (to warn) — originally not a creature but a sign from the gods.
An imaginary creature that is large, ugly, and frightening; also, a person of inhuman cruelty or wickedness.
From Old French 'monstre,' from Latin 'mōnstrum' (a divine omen, a portent, a monster, an unnatural thing), from 'monēre' (to warn, to advise, to remind), from PIE *men- (to think). In Roman religion, a 'mōnstrum' was not primarily a terrifying creature but a divine warning — an unnatural birth, a prodigy, a sign sent by the gods to alert humans to something amiss. Deformed animals, two-headed
A 'monster' is etymologically a warning from the gods — Latin 'mōnstrum' (a divine portent) from 'monēre' (to warn). The same root produced 'monitor' (one who warns), 'admonish' (to warn against), 'premonition' (a forewarning), and 'demonstrate' (to point out clearly). Every monster was originally a message.