What sea monster was sent to devour Andromeda?

The sea monster sent to devour Andromeda was Cetus.

In the most familiar version of the myth, Queen Cassiopeia claimed that she or her daughter Andromeda was more beautiful than the Nereids. Poseidon punished this boast by sending Cetus to ravage the coast of Aethiopia. An oracle declared that Andromeda had to be sacrificed, so her parents chained her to a rock beside the sea.

Perseus encountered Andromeda while returning from his quest for Medusa. He killed Cetus and rescued her, later marrying her. Accounts differ about the method: some say Perseus used his sword, while others say he turned the monster to stone with Medusa’s severed head.

Cetus is also a descriptive Greek word rather than a uniquely detailed species. The ancient term kētos could mean a huge sea monster or sea serpent, and artistic depictions vary widely. The same myth gave its name to the constellation Cetus, traditionally represented as a whale-like monster.