Antaeus

Antaeus, in Greek mythology, was a son of Poseidon and Gaia. He was extremely strong as long as he remained in contact with the ground (his mother). He challenged all comers to wrestling matches, beat them and killed them until Heracles arrived, discovered his secret and destroyed him.

Pliny, quoting Euanthes[?], says (Hist. Nat. viii. 22) that a man of the Antaeus family was selected by lot and brought to a lake in Arcadia, where he hung his clothing on an ash tree and swam across. This resulted in his being transformed into a wolf, and he wandered in this shape nine years. Then, if he had attacked no human being, he was at liberty to swim back and resume his former shape.

Ovid IX, 184.