In Greek mythology, Cetus was the sea monster Perseus defeated while rescuing Andromeda.
Cetus was sent by Poseidon after Cassiopeia offended the sea god by boasting about her daughter Andromeda. To save the kingdom, Andromeda was exposed on a rocky shore as a sacrifice. Perseus arrived after killing Medusa and confronted the monster.
Versions differ about exactly how Perseus defeated Cetus. Some say he killed it with his sword, while others emphasize the petrifying power of Medusa’s head. Cetus is also the source of the scientific name for the constellation Cetus, although the constellation’s modern identity is usually translated as a sea monster or whale. Cetus should not be confused with Scylla, Charybdis, or the Hydra, which belong to different mythological episodes.