Ceto

In Ancient Greek mytholoy Ceto is the name of a sea monster;often represented as a whale, or a large fish. She is the daughter of Pontos and Gaia. Together with her brother Phorcys she bore the Gorgons, and Graeae, Echidna and Ladon.
Note there is also a minor oceanid called Keto. There are also various mythological beings referred to as "ketos" (plural "ketea"); this is a general term for "sea monster" in Ancient Greek.