Phylacus was king of Phylace in Thessaly and a son of Deïon and Diomede. His wife was Clymene, who bore him Iphiclus and Alcimede, the mother of Jason. Phylacus was famous for his fine herds that Melampus tried to steal. It can therefore be appreciated that for him his name, meaning 'guardian', 'watcher' (phylax), was specially significant.