In ancient Greek, ketos denotes a large fish, a whale, a shark, or a sea monster. The sea monsters slain by Perseus and Heracles were each referred to as a cetus by ancient sources. The term cetacean (for whale) originates from cetus. In Greek art, cetea were depicted as serpentine fish. The name of the mythological figure Ceto is derived from ketos. The name of the constellation Cetus also derives from this word.
When Queen Cassiopeia boasted that her daughter Andromeda was more beautiful than the Nereids, this invoked the wrath of Poseidon who sent the sea monster Cetus to attack Ethiopia. Upon consulting a wise oracle, King Cepheus and Queen Cassiopeia were told to sacrifice Andromeda to Cetus. They had Andromeda chained to a rock near the ocean so that Cetus could devour her. Perseus found Andromeda chained to the rock and learned of her plight. When Cetus emerged from the ocean to devour Andromeda, Perseus managed to slay it. In one version, Perseus drove his sword into Cetus ' back. In another version, Perseus used Medusa 's head to turn Cetus to stone.
The Cetus is commonly depicted as fish-like, serpentine, with a long muzzle. Alternate depictions may include long ears, horns and legs instead of flippers. It is often depicted fighting Perseus or as the mount of a Nereid.
In the Bible, in Jonah 2:1 (1:17 in English translation), the Hebrew text reads dag gadol, which literally means "great fish". The Septuagint translates this phrase into Greek as mega ketos. The term ketos alone means "huge fish", and in Greek mythology the term was closely associated with sea monsters. Jerome later translated this phrase as piscis grandis in his Latin Vulgate. However, he translated the Greek word kētos as cetus in Gospel of Matthew 12:40: "For as Jonah was three days and three nights in the belly of a huge fish, so the Son of Man will be three days and three nights in the heart of the earth."
Cetus is commonly used as a ship 's name or figurehead denoting either a ship unafraid of the sea or a ruthless pirate ship to be feared. Cetus (and its translations) are also viewed as misfortune or bad omen by sailors. Superstitious sailors believed in a cetus as the bringer of a great storm or misfortune on the ship. They associated it with lost cargo, the presence of pirates, or being swept off course, and avoided any talk of it aboard ship.