Zetus (or Zetes) with Calais, was one of the twin sons of Boreas and Orithyia. His name is best derived from zao, 'to blow', especially in view of his parents' 'windy' names (see them) and his own function, with Calais, as a winged chaser of the Harpies when these monsters were plaguing their brother-in-law, king Phineus. Even an origin based on zeleo, 'to search out', would also be fitting. According to an ancient theory, Calais' name meant 'he who blows gently', while Zetus' meant 'he who blows hard'.