In Greek mythology, the Old Man of the Sea was a primordial figure who could be identified by several names, Proteus or Nereus or Pontus
Nereus was the old man of the sea, and the god of the sea's rich bounty of fish. He dwelt in the depths of the Aegean with his wife Doris and fifty Nereid daughters. Like many of the other sea-gods Nereus was a master shapeshifter, and spoke with prophetic voice. It was for his knowledge that Herakles once wrestled him, in search of directions to the land of the Hesperides.
Nereus was depicted in ancient Greek vase painting as an old man with a wooden staff accompanied by a host of his Nereid daughters. Sometimes he had a coiling fish tail in place of legs (like the image right). But unlike other similar gods, such as Triton and Akheloios, Nereus was almost always depicted clothed in a chiton. (Source:theoi.com)