In modern Genre Fantasy, monsters are a writer's convenience (> Plot Coupon) for placing yet another obstacle in the way of the hero's Quest. J R R Tolkien used them to particular good effect in The Lord of the Rings (1954-55), also creating new monsters, like the Balrog, the giant spider (> Spiders) Shelob, and the many-tentacled Watcher in the Water. More traditional monsters are often the purpose of a quest and form a fundamental part of folk tradition. The commonest monsters are Dragons and Giants, which feature heavily in Legends, Myths, Folktales and medieval Romance. Diverse other monsters are treated separately in this encyclopedia under Animals Unknown to Science, Godzilla (> Godzilla Movies), Gorgons, Imaginary Animals, Minotaur, Monster Movies, Mythical Creatures, Sea Monsters, Serpents, Trolls, Vampires, Werewolves, Worms and Zombies. Most monsters instil dread and are killed to our hero's greater glory, though they may not be inherently evil; e.g., in Beowulf, Grendel's mother may wreak havoc, but she is merely avenging the killing of her child. Similarly, King Kong may through sheer size cause much destruction. The Frankenstein monster has not the capacity to be either good or evil: its "sin" is that its mere appearance causes revulsion (or, to read more deeply, that it has become the repository for its creator's well justified shame), and this leads to its performing evil acts.
Humans can be monsters in more subtle ways, where the monstrous is not apparent on the surface. Most monster-humans have become so through either Possession or dabbling in the occult. Some monstrous women may be Femmes Fatales. Historical examples of both who appear in Horror are Gilles de Rais (1404-1440), the Marquis de Sade and Elizabeth de Báthory (? -1614).
A different definition of "monster" might illuminate. The word comes from the Latin monstrum, meaning something Marvellous to be treated as a warning or Portent. Many of the Roman annalists noted the birth of freaks or monsters (animal or human) each year as if their occurrence was in some way related to the dread events of those years. The appearance of monsters thus indicates Wrongness, and is used in this form, alongside other cataclysms, as a warning in "The Call of Cthulhu" (1928 WT) by H P Lovecraft.
Children's Fantasy has some of the best monsters because the authors can play more to a child's imagination. Excellent examples appear in the poems and stories of Lewis Carroll, Edward Lear and Terry Jones, the Narnia books by C S Lewis, The Phantom Tollbooth (1961) by Norton Juster and The Neverending Story (1979) by Michael Ende. R Chetwynd-Hayes delights in creating monsters; a selection is in The Monster Club (coll 1976).
Anthologies include Monsters Galore (anth 1965) ed Bernhardt J Hurwood (1926-1987), A Walk with the Beast (anth 1969) ed Charles M Collins, Monsters, Monsters, Monsters (anth 1977) ed Helen Hoke (1903-1990), Bestiary! (anth 1985) and others in the series by Jack Dann and Gardner Dozois, and The Monster Book of Monsters (anth 1988) ed Michael O'Shaughnessy (1965- ). Interesting studies include The Book of Beasts (1954) by T H White, Fictitious Beasts: A Bibliography (1961) by Margaret W Robinson, The Book of Imaginary Beings (1967; rev trans 1969) by Jorge Luis Borges, and The Magic Zoo (1979) by Peter Costello (1946- ). [MA]