This is the "Panel of Perseus and Andromeda", inv. 849/a. From Samandağı, second-third century AD. From the Enc. Britt.: (Andromeda:) in Greek mythology, beautiful daughter of King Cepheus and Queen Cassiope of Joppa in Palestine (called Ethiopia) and wife of Perseus. Cassiope offended the Nereids by boasting that Andromeda was more beautiful than they, so in revenge Poseidon sent a sea monster to devastate Cepheus' kingdom. Since only Andromeda's sacrifice would appease the gods, she was chained to a rock and left to be devoured by the monster. Perseus flew by on the winged horse Pegasus, fell in love with Andromeda, and asked Cepheus for her hand. Cepheus agreed, and Perseus slew the monster.
Note the unmonstruous monster.