In Ancient Greek ketos, Latinized as cetus, is any huge sea monster. According to the mythology, Perseus slew a cetus to save Andromeda from being sacrificed to it. The term cetacean 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.

Sea monsters are beings from folklore believed to dwell in the sea and are often imagined to be of immense size. Marine monsters can take many forms, including sea dragons, sea serpents, or tentacled beasts. They can be slimy and scaly and are often pictured threatening ships or spouting jets of water. The definition of a "monster" is subjective; further, some sea monsters may have been based on scientifically accepted creatures, such as whales and types of giant and colossal squid.