The monstrous creature that Perseus defeated to save Andromeda is the Kraken(or sea monster). According to mythology, Andromeda was chained to a rock as a sacrifice to appease Poseidon’s wrath. Perseus, armed with a reflective shield and the head of Medusa, fought off the creature, thus rescuing her. This tale highlights themes of heroism, bravery, and the intervention of fate. Perseus' victory not only saved Andromeda but also reinforced his status as one of the greatest heroes in Greek mythology.