Cetus

Cetus was a sea monster that Poseidon sent to punish Cassiopeia, wife of King Cepheus of Joppa, and mother of Andromeda. Cassiopeia had boasted that her daughter was more beautiful than the Nereïds, the sea nymphs.

The city would be spared if they sacrificed Andromeda to Cetus. The unfortunate princess was chained to the rock on the cliff. Perseus told the king that he would save her if he was allowed to marry Andromeda. Perseus either killed Cetus with a sickle of adamant, given to him by Hermes, or he used the head of the Gorgon Medusa to turn the sea monster into stone.