At the birth of Poseidon when the babe was shown to him by Rhea, his wife.
Rhea
No myth gives the birth-day or year of Poseidon's life.
The hometown of Poseidon was Atlantis, mythical place.
Poseidon wooed Medusa who gave birth to Pegasus, who technically is Poseidon's offspring.
Cronos & Rhea
Quite possibly, as Cronus the father of Poseidon swallowed him shortly after birth.
Poseidon's childhood was like no other. Poseidon's father was afraid that one of this sons were going to dethrone him so he would swallow them at birth Aracdains say that when Poseidon was born his mother declared to her husband that she had giving birth to a horse. by" Keylee Floyd
Rhea gave birth to him normally, but than Cronus swallowed him.
Poseidon's childhood was like no other. Poseidon's father Kronos was afraid that one of his children were going to dethrone him so he would swallowed them at birth. Some myths say that when Poseidon was born, his mother (Rhea) declared to her husband that she had given birth to a horse. Later, Poseidon's younger brother Zeus (who did not get swallowed) tricked Kronos into spitting out his children Poseidon, Hades, Hestia, Demeter, and Hera.
There is no story of his birth. We simply know he was born to Kronos and Rhea...and then Kronos ate him...
Poseidon was born by Rhea, his mother, and swallowed and later spat up again by his father Cronus.