Cetus, the whale or sea-monster:

An ancient constellation that is part of the story of Perseus and Andromeda. Cetus was the monster that Andromeda was to be sacrificed to in order to ease the anger of the sea god Poseidon. Cetus is an extremely large constellation that has been represented as both a sea-monster or as simply a whale. The constellation can be found by looking below Pisces the fish.