Byzas

In Greek legend, Byzas (Βύζας, ) was the eponymous founder of Byzantium (Greek: Βυζάντιον), the city later known as Constantinople and Istanbul.

Founding of Byzantium
Byzas, son of King Nisos (Greek Νίσος), was a Greek colonist from the Dorian city of Megara who consulted the oracle of Apollo at Delphi. The oracle instructed Byzas to settle opposite from the "Land of the Blind". Leading a group of Megarian colonists, Byzas found a location where the Golden Horn, a great natural harbour, meets the Bosphorus and flows into the Sea of Marmara, opposite Chalcedon (modern day Kadıköy). He determined the Chalcedonians must have been blind not to recognize the advantages the land on the European side of the Bosphorus had over the Asiatic side, and in 667 BC founded Byzantium on the European side, thus completing the oracle's quest.

Son of Poseidon
In Greek mythology, Byzas was a son of Poseidon by Keroessa.

Zeus fell in love with Io, the daughter of Inachus, King of Argos and god of the river of Argos. Zeus temporarily transformed his mistress into a heifer in order to protect her from the wrath of his wife Hera. In her wanderings, Io crossed the Bosphorus, giving the strait its name ( βοός πόρος - boos poros, which is Greek for cow-ford). After reassuming her original form, she gave birth to a girl, Keroessa.

Keroessa later bore a son to Poseidon, elder brother of Zeus and lord of all waters from the Pillars of Hercules to the Hellespont. This son, Byzas the Megarian, in time became the founder of Byzantium and named the Golden Horn (Greek Χρυσοκέρας (Khrysokeras or Chrysoceras) after his mother. Some sources say that Byzas was brought up by the naiad Byzia and married Phidaleia, daughter of King Barbyzos.