King Neptune was the father of Poseidon in Disney's Greek Mythology. "Neptune" was also Poseidon's Roman counterpart. He appeared in "King Neptune (Silly Symphonies)", angry because sailors had been hurting his mermaids. He was the grandfather of Triton, and the great-grandfather of Ariel and her sisters.
He is probably based on Oceanus, a Titan from before the Gods, because he was before Poseidon (a God). Also, Oceanus was Poseidon's uncle, and here he is his father.