Adrienne Mayor demonstrates that at least one Greek monster was conjured up from the fossil bones of an extinct creature. Far and away, however, most Greek monsters began their life in folktale or were borrowed from barbarian (that is, non-Greek) peoples.

We know of the earliest monsters from storytellers and poets, not artists. Only after the eighth century B.C. did artists begin depicting monsters killed by mythical heroes. These painters and sculptors were obliged to invent images for monsters that had hitherto been just names, and whose appearance had never been described in any detail. Greek ingenuity in devising shapes for the monsters of their myths is nowhere better demonstrated than in their representations of the sea monster, or ketos, which from an early date appeared in stories involving heroes rescuing princesses, such as the legend of the Monster of Troy—though the sea monster must also have been a popular figure in tales told by mariners sailing the Mediterranean.

The earliest representations show the sea monster (or monster-like deity) with lion forelegs, a fish tail and sometimes multiple heads. The sea-nymph Thetis, mother of Achilles, was often depicted attended by such a lion-fish, especially when shown wrestling with Peleus, her future husband. An example of the lion-fish also appears on the sixth-century B.C. vase shown at the end of the main article, which shows the Monster of Joppa (modern Jaffa, near Tel Aviv) being killed by Perseus for menacing the princess Andromeda.

By the fifth century B.C. a new ketos had evolved, one inspired by sheer fantasy rather than by fossils or Near Eastern art (whence the Greeks got their sirens, sphinxes and griffins). This monster has a long snout, ears and gills, and flippers or sometimes the original lion forelegs. Its neck is slim, its belly huge, and its fishy tail long and convoluted—as in a fourth-century B.C. vase in the Getty Museum, showing Eros riding a sea monster being attacked by Perseus (below). The ingredients for this sea monster are of mixed origin—crocodile, sea horse, griffin, pig, lion and fish—and the resulting beast resembles the much-earlier Babylonian dragon.

This new ketos had many roles, not least a purely decorative one. Its head could serve as a figurehead on a warship (instead of the usual boar’s head). Troops of them are sometimes depicted carrying Thetis and her sister nymphs over the seas as they bring new armor to Thetis’s son, Achilles, at Troy. These monsters are probably seen at their best in the great colorful mosaics of Roman-period Tunisia and Antioch, or on Roman silver vessels.

Nor has the Greek ketos been forgotten. For instance, the “great fish” that swallows the biblical Jonah, who sailed from Joppa, has frequently been represented as a Greek sea monster—as on a third-century A.D. sarcophagus (see detail above) in the Vatican Museum, and in a 12th-century A.D. mosaic (below) on an ambo (lectern) in the Duomo at Ravello, Italy. For centuries, moreover, depictions of St. George fighting the dragon were based on the iconography of Perseus rescuing Andromeda from a ketos. In medieval art, the ketos often became a symbol of the sea (Thalassa) giving up her dead at the last trump.

Perhaps the very artificiality of the ketos guaranteed its longevity, allowing it to change its shape to fit the needs of the imagination of the day. If, as in medieval art, Noah did not allow these mixed creatures onto the Ark, the ketos had no need of rescue from the Flood, for its future was assured.