Akemi Hirokawa

Edited version for theaters of the eponymous 1972 show, about Triton, a sole survivor who seeks vengeance against evil sea demigod Poseidon and his monsters who eradicated his clan of good mermen and usurped their throne. (imdb)

5000 years ago, evil demigod Poseidon and his minions attack the good mermen. A dolphin saves one of them, baby Triton. Later, as a young boy, Triton decides to avenge his family and end the reign of evil Poseidon and his monsters.