Medusa had given birth to a baby because of Poseidon, he raped her in Athena's temple, causing Athena to curse Medusa. When the news that she had given birth reached Olympus, all the gods, even Poseidon, were surprised. zeus: it looks like someone here is dad, right, Poseidon...

zeus murmured while looking at Poseidon.

while hades held the baby because on the same day that medusa gave birth she abandoned the baby near olympus