Origins
The word 'monster' conceals one of the most surprising etymological revelations in the language: behind every dragon, every beast, every creature of nightmare stands not a roar but a message. Latin 'mōnstrum,' from which the English word descends, meant first and foremost a divine warning — an omen, a portent, a sign from the gods that something in the natural order had gone wrong.
The word enters English in the late thirteenth century from Old French 'monstre,' from Latin 'mōnstrum' (a divine omen, a portent, a prodigy, an unnatural thing, a monster). Latin 'mōnstrum' derives from 'monēre' (to warn, to advise, to remind), from PIE *men- (to think, to have in mind). The connection is direct: a 'mōnstrum' was something through which the gods 'monēre' — warned or reminded — human beings. In Roman religious practice, prodigies such as deformed births, unusual natural phenomena, and strange animal behavior were treated as 'mōnstra,' signs that the pax deōrum (peace of the gods) had been disturbed and that remedial rituals were needed.
The PIE root *men- (to think) is one of the most productive in the Indo-European family. Through Latin 'monēre,' it produced 'monitor' (one who warns — originally a school prefect tasked with keeping order), 'monument' (from 'monumentum,' a reminder, a memorial), 'admonish' (from 'admonēre,' to warn toward), 'premonition' (from 'praemonēre,' to forewarn), 'summon' (from 'submonēre,' to secretly remind, to call forth), 'remonstrate' (from 'remōnstrāre,' to show again, to protest by pointing out), and 'demonstrate' (from 'dēmōnstrāre,' to point out, to show clearly). Through Latin 'mēns' (mind), from the same PIE root, it gave us 'mental,' 'mention,' 'comment,' and 'dementia.' Through Sanskrit 'manas' (mind), it appears in 'mantra.'
French Influence
The semantic shift from 'divine warning' to 'frightening creature' happened gradually. The unnatural births and prodigies that Romans classified as 'mōnstra' were themselves strange, disturbing, and often repulsive. Over time, the quality of the sign — its grotesque, fear-inducing nature — came to dominate over its communicative function. By Late Latin and Old French, 'mōnstrum/monstre' could refer to any creature of unnatural appearance or size, whether or not it was understood as a divine message. By the time the word was fully naturalized in English, the omen had vanished and only the beast remained.
The related word 'demonstrate' preserves the warning-root in a different register. From Latin 'dēmōnstrāre' (to point out, to show clearly, from 'dē-' fully + 'mōnstrāre' to show, itself from 'mōnstrum'), it retains the core sense of making something visible and unmistakable — showing a sign. A demonstration is, etymologically, the display of a portent. The scholarly word 'remonstrate' (to protest, to object by showing reasons) keeps the same sense: to show again, to re-present the warning.