Poseidon

From Tardis Wiki, the free Doctor Who reference
For the planet of the same name, see Neptune.

Neptune was king of Earth's oceans. He and his mermaid servants lived in a castle made of sand located on a South Seas island. After sending a wave to move the Doctor, John and Gillian out of the way of a volcanic eruption, he provided them with a great feast. Before they left, he gave a pearl necklace to Gillian and a golden shell to John. (TVC: Guests of King Neptune)

King Neptune may have been connected to the Immortals who posed as other Greek gods.
Poseidon