1. An imaginary or legendary creature, such as a centaur or Harpy, that combines parts from various animal or human forms.
2. A creature having a strange or frightening appearance.
3. An animal, a plant, or other organism having structural defects or deformities.
4. Pathology. A fetus or an infant that is grotesquely abnormal and usually not viable.
5. A very large animal, plant, or object.
6. One who inspires horror or disgust: a monster of selfishness.

Middle English - from Old French monstre, from Latin mōnstrum, portent, monster, from monēre, to warn.

The English word monster can be traced back to various etymological roots. The Latin 'monstrum' means 'that which teaches', and there is also another connection to 'monstrare', to show. Both Latin words derive from the same base 'monere', to warn. 

The theme of teaching or guiding is thus implicit in the etymology, with the English word 'demonstrate' turning out to be a cousin of 'monster' in that the Latin 'demonstratum' is a past participle of 'demonstrare', which means 'to point out, indicate, show or prove'.  Ancient peoples considered the birth of "freaks" representations of the wrath of the gods, usually omens.

It suggests that the ancient pagan (as in non-Judeo-Christian-Islamic) world may well have had a somewhat different view of monsters than 'we' in the contemporary Judea-Christian cultures of Western Europe do.

Occasionally, there are modern monsters who act out of legitimate motives and their monstrous appearance leads to serious misunderstandings. Some well known examples are King Kong , Frankenstein's Monster, and the Horta in the Star Trek episode, "The Devil in the Dark".