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.

Corithian vase showing Perseus, Andromeda and Ceto/Ketos.

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.^[1]