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early 14c., "malformed animal or human, creature afflicted with a birth defect," from Old French monstre, mostre "monster, monstrosity" (12c.), and directly from Latin monstrum "divine omen, portent, sign; abnormal shape; monster, monstrosity," figuratively "repulsive character, object of dread, awful deed, abomination," from root of monere "warn" (see monitor (n.)). Abnormal or prodigious animals were regarded as signs or omens of impending evil. Extended by late 14c. to imaginary animals composed of parts of creatures (centaur, griffin, etc.). Meaning "animal of vast size" is from 1520s; sense of "person of inhuman cruelty or wickedness" is from 1550s. As an adjective, "of extraordinary size," from 1837. In Old English, the monster Grendel was an aglæca, a word related to aglæc "calamity, terror, distress, oppression."
monster mon·ster (mŏn'stər)
An animal, a plant, or other organism having structural defects or deformities.
A fetus or an infant that is grotesquely abnormal and usually not viable.
in biology, an embryo, a newborn animal, or young plant that is grossly deformed. The defects may be genetic (i.e., inherited) or result from such influences as drugs, X rays, or diseases. Two main types of monster are recognized: those with defective or excessive growth of body parts and those with partial or complete doubling of the body on one of its axes. The repetition or absence of body parts such as fingers or toes are frequent anomalies in humans and other mammals.