Cygnus could possibly be the son of Poseidon. When Zeus, Hades, and Poseidon overthrew their father, they drew lots to see who would rule what. Poseidon won the seas, Hades won the Underworld, and Zeus won the Heavens. Poseidon later decided he would like to rule the earth. A contest was held in which Athene won the Lands by a single vote.

Cygnus, Poseidon's son, had been abandoned at birth on the beach. A swan took pity on him and flew down to take care of the infant. Although Cygnus became a king of a realm north of Troy, he was not a good king. He sent his own children to be drowned when his wife professed her love for his son. Then he killed his wife when he found out she had lied to him. When Achilles of Troy attacked the city, Cygnus was choked to death. Poseidon was unhappy by this and so he turned Cygnus into a swan.