thumb|Ancient vase motif depicting the Greek hero [[Perseus fighting a Cetus sea serpent]]
thumb|Ancient Corinthian vase depicting [[Perseus, Andromeda and Ketos (Names are spelled in the archaic Corinthian variant of the Greek alphabet.)]]
thumb|Ancient Greek mosaic in [[Caulonia (ancient city)|Kaulon of a cetus sea serpent]]
In Greek mythology, a Cetus () is a large sea monster. Perseus slew a cetus to save Andromeda from being sacrificed to it. Later, before the Trojan War, Heracles also killed one to rescue Hesione. The term cetacean (for whale) derives from cetus. In Greek art, ceti were depicted as serpentine fish. The name of the mythological figure Ceto is derived from kētos. The name of the constellation Cetus also derives from this word.
Etymology
In Ancient Greek ketos (, plural kete/ketea,
Depictions
A cetus was variously described as a sea monster or sea serpent. Other versions describe a cetus as a sea monster with the head of a wild boar
Greek mythology
thumb|Ritual [[stone palette a Nereid (Sea Nymph) and a Cherub riding a Sea Monster (Ketos). Gandhara.]]
Cetus are often depicted fighting Perseus or as the mount of a Nereid.
Queen Cassiopeia boasted that she and her daughter Andromeda were more beautiful than the Nereids, invoking the wrath of Poseidon who sent the sea monster Cetus to attack Æthiopia. Upon consulting a wise oracle, King Cepheus and Queen Cassiopeia were told to sacrifice Andromeda to the Cetus. They had Andromeda chained to a rock near the ocean so that the cetus could devour her. After finding Andromeda chained to the rock and learning of her plight, Perseus managed to slay the Cetus when the creature emerged from the ocean to devour her. According to one version, Perseus slew Cetus with the harpe lent to him by Hermes. According to another version, he used Medusa's head to turn the sea monster to stone.
In a different story, Heracles slew a Cetus to save Hesione.
A Cetus had also been portrayed to support Ino and Melicertes when they threw themselves into the sea instead of a dolphin to carry Palaemon.
In both cases, the ruler annoyed Poseidon.
Etruscan mythology
In Etruscan mythology, the Cetea were regarded as psychopomps, being depicted frequently on sarcophagi and urns, along with dolphins and hippocamps.
Furthermore, the Etruscan deity Nethuns is sometimes shown wearing a headdress depicting a Cetus.
Bible and Jewish mythology
The tannin sea monsters
The monster tannin in the Hebrew Bible has been translated as Greek kētos in the Septuagint, and cetus in the Latin Vulgate.
Tanninim () (-im denotes Hebraic plural) appear in the Hebrew Book of Genesis, Exodus, Deuteronomy, Psalms,