The monstrous creature that Perseus defeated to save Andromeda is the sea monster known as Cetus. According to mythology, Andromeda was chained as a sacrifice to this beast due to her mother's boastful claim about her beauty. Perseus, equipped with gifts from the gods, including a reflective shield and a winged sandals, successfully slays Cetus, showcasing his bravery and heroism. This tale highlights themes of rescue, valor, and the triumph of good over evil.