Nereus

Nereus was the son of Pontus and Thalassa. He was a sea-god and the husband of Doris. He was the father of the 50 Nereids. He was a famed shape-shifter and was commonly called the "Old Man of the Sea", along with his father and Proteus. He was the father-in-law of Peleus (and grandfather of Achilles) by his daughter Thetis and father-in-law of Poseidon (and grandfather of Triton) by his daughter Amphitrite.