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.
2. An animal, a plant, or other organism having structural defects or deformities.
3. Pathology A fetus or an infant that is grotesquely abnormal and usually not viable.
4. A very large animal, plant, or object.
5. One who inspires horror or disgust: a monster of selfishness.
---American Heritage Dictionary