In Greek mythology, Poriclymenus (or Periclymenus) referred to two different people.
Poriclymenus was a son of Poseidon and Chloris and would-be murderer of Amphiaraus in the battle for Thebes. Amphiaraus was swallowed by the earth before he could kill him though.
Poriclymenus was a son of Nereus and one of the Argonauts. Poseidon gave him the ability to shapeshift in various animals. He was killed by Heracles at Pylos.
Ovid XII, 556