A monster (Latin monstrum portent or unnatural event, from moneo I warn) is either:
Monsters are a frequent device of science fiction. In such works, their purpose is almost always to warn mankind not to play with forces beyond his comprehension or ability to control. Some stories feature monsters that are works of man. The novel Frankenstein, or the Modern Prometheus by Mary Shelley was probably the first of these.
In medicine, a "monster" is usually an infant suffering from congenital abnormalities that often kill or cripple the patient or threaten to make the patient unsightly in the extreme and likely to elicit only revulsion from all who look upon him, absent a corrective measure such as plastic surgery.
The word monster could also apply to the field of biology, to encompass any hybrid creature that a biologist might build in his laboratory. Most biologists dislike this term, because of its connotation of horror—which is, strictly speaking, the state of shuddering from fright (from the Latin horreo I shudder) or something that makes one shudder. Yet the word monster, if one takes the word at its original meaning, might suggest a warning against a treatment of the stuff of human life in an ethically or morally reprehensible manner.