κῆτος (ketos), Latinized as Cetus, was the catch-all definition for "sea monsters" in Ancient Greek mythology. In modern Greek, ketos is the word for whale. The best-known "Cetus" in mythology was the one which threatened Princess Andromeda of Aethiopia. The princess' mother Queen Cassiopeia had boasted that she and her daughter were more beautiful than the Nereids, which invoked the wrath of Poseidon, who sent Cetus to attack Aethiopia. Upon consulting the Delphic oracle, King Cepheus and Queen Cassiopeia were told to sacrifice Andromeda to Cetus. They had Andromeda chained to a rock near the Mediterranean Sea, so that Cetus could devour her. Andromeda was rescued by the intervention of Prince Perseus of Argos, who managed to slay Cetus when the creature emerged from the ocean to devour Andromeda. In different versions, Perseus killed Cetus with a sword, or used Medusa's head to turn the monster to stone.

It is believed that the concept of Cetus (and its analogs in other cultures) grew out of poorly transmitted eyewitness account of whales, oarfish, and other large sea creatures, and/or the discovery of fossilized Mesozoic animals such as Plesiosaurus and Mosasaurus.

Cetus in "Myth Manners' Guide to Greek Missology"[]

King Acrisius of Argos was told by an oracle that his grandson Perseus would one day kill him, so he tried to have the young man eliminated in a way that would not get blood on the king's own hands. He thus had Perseus tied to a rock where a sea serpent was sure to emerge. The intervention of the warrior princess Andromeda upset Acrisius' plans. With a sword, Andromeda hacked the beast's head off, although it was so stupid that it did not understand its own expiration for a long time after the fact, and kept on fighting. Andromeda then rescued Perseus.[1]

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  1. Counting Up, Counting Down, pgs. 278-280, tpb.