Herakles rescues a king’s daughter, Hesione, from a raging sea monster sent by a spurned Poseidon. The myth mimics the well-known story of Perseus and Andromeda, but was never as popular in art and literature.
You can just see Herakles battling the monster at the bottom left of this ancient fresco.
Fresco Fragment with Herakles and Hesione, about A.D. 70, Roman. The J. Paul Getty Museum