Iphimedeia

Therefore Pollux was permitted to divide his immortality with his brother Castor, dying and living alternatively.

There was Iphimedia, who bore two sons to Neptune, that were giants, Otus and Ephialtes.

earth in her prodigality never nourished bodies to such portentous size and beauty as these two children were of except orion at nine years old they had imaginations of climbing to heaven to see what the gods were doing they thought to make stairs of mountains and were for piling ossa upon olympus and setting pelion upon that and had perhaps performed it if they had lived till they were striplings but they were cut off by death in the infancy of their ambitious project phaedra was there and procris and ariadne mournful of theseus desertion and myrrha and clymene and eriphyle who preferred gold before wedlock faith but now came a mournful ghost That late was Agamemnon, the son of Atreus, the mighty leader of all the host of Greece and their confederate kings that warred against Troy.

He came with the rest to sip a little of the blood at that uncomfortable banquet.

In another instance, Poseidon turned himself into a ram and kidnapped Theophane, taking her to an island where he fathered the ram with the Golden Fleece, the same Golden Fleece that would find its way to Colchis and be the MacGuffin the Argonauts would go after.

He fathered the Aloadae, two giants, when the Princess Iphimedeia sat down in the ocean, because that's apparently a thing.

The giants would go on to storm Olympus and lose.

Poseidon claims parentage of Theseus, but that's dubious given how successful Theseus was.