Proteus, the so-called Old Man of the Sea, is a prophetic sea divinity, son of either Poseidon or Oceanus. He usually stays on the Island of Pharos, near Egypt, where he herds the seals of Poseidon. He will foretell the future to those who can seize him, but when caught he rapidly assumes all possible varying forms to avoid prophesying. When held fast despite his struggles, he will assume his usual form of an old man and tell the future.
Odyssey 4. 351-570, particularly 455-458.