Today I will discuss some other Deities who are related to Poseidon in some way or another.
Being the King of the Seas and Waters of the World, this makes all Sea and Water Deities and Spirits his subjects. In the previous two posts, I made mention of the children of Nereus several times, from Nerites as Poseidon’s lover to Amphitrite the Nereïd as his wife and Queen. Nereus himself is also closely associated with Poseidon. He is called the Old Man of the Sea (Halios Geron) and had the power of prophecy. In order to get him to prophesy to you, one had to capture him and hold him as he frantically tried shifting his shape into various creatures in an attempt to escape. Once he realises it is futile he shifts to his original form and will answer your questions truthfully. In this, he is extremely similar to another Sea Deity, Proteus, who is also called Old Man of the Sea and can prophesy. Proteus also serves as the shepherd of Poseidon’s flocks of seals. Going Back to Nereus, he is the father-in-law of Poseidon because of the laters marriage to Amphitrite, daughter of Nereus. He is himself the son-in-law of Okeanos, because of his marriage to the Okeanid Doris, daughter of Okeanos and Tethys.
Other Sea Deities who are subject to Poseidon’s rule are Okeanos, from whom all Rivers flow, Tethys, Pontos, Thalassa/Thalatta, and countless other Sea Deities, Spirits, Nymphs, etc. The Rivers also are subjects of Poseidon, the main River Gods that feature in ancient Myths are Akheloios, Inakhos, Alpheios, Eridanos, Akheron, etc. Pretty much all Rivers are his Sons and all Springs his Daughters. For a more complete list of River Deities than I can provide here without getting tedious, you can check this page on theoi.com.
Yesterday, I pretty much stuck with Deities whom Poseidon was very directly related to, mainly his genealogy and his offspring. But he also has relationships with his various nieces and nephews. Prominently among them, there is Athena, daughter of Zeus sort-of-by-Metis and sort-of-by-Himself; with her, Poseidon competed for the land of Attika, which sought a Patron Deity for the newly established polis. In this contest, both Deities were allowed to offer a gift to the polis, the people would judge it and then the King, Kekrops, would make his decision. Poseidon, in the version I am familiar with, created two springs on or near the Acropolis, one hot spring and one cold spring. However, according to Theoi.com, the gift Poseidon produced was the very first horse. This would be the first time I heard of this being the gift Poseidon produced, but Theoi.com cites the following sources: Lucan, Phars. vi. 396, &c.; Hom. Il. xxiii. 277; Apollod. iii. 13. § 5. Athena, on the other hand, planted the very first olive tree and taught its cultivation to the people. Kekrops decided that Athena was the victor, for the olive could be used as food, for its oil, and perhaps most importantly as a trade good that would bring wealth to Athens. Poseidon in his anger over the loss either flooded parts of Attika or caused a drought in the region. Nevertheless, Poseidon is also one of the main Deities in Athenian public worship, since his blessing is so vital for the marine trade on which the polis relies. The springs he made on the Acropolis were integrated into what is called the Erekhtheion, one of the oldest sanctuaries on the Acropolis.
Another child of Zeus with whom Poseidon is associated is Apollon, with whom Poseidon laboured to build the great walls of Troy. Laomedon, King of Troy at the time, refused their payment once the walls were finished, going so far as to dismiss them with threats. For this Poseidon was enraged and sent a sea monster to devour Laomedon’s daughter, though it was killed by Herakles before it could perform its appointed task. Ever after, Poseidon held a grudge against the Trojans and indeed fought alongside the Hellenes in the Trojan War. Though it did sting him that the great walls he helped build would be wrecked by the Hellenes when finally the Hellenes were victorious. Poseidon also seems to have, at one point, shared ownership of Delphoi with Gaia, but Apollon is said to have traded him Kalauria in exchange for Poseidon’s stake in Delphoi.
Poseidon seems to have a knack of disputing ownership of lands and cities with other Deities, which is new information to me. He had another spat with Athena over Troizene until Zeus himself commanded hey share it equally (Paus. ii. 30. § 6). With Helios, he disputed ownership of Korinthos, where the Acrokorinthos (the acropolis of Korinthos) was awarded to Helios, but the surrounding lands and the Isthmos, in particular, were awarded to Poseidon (Paus. ii. 1. § 6.). He even disputed ownership of the Argolis with Hera, whose ownership was awarded to her by the River Gods Inakhos, Kephissos, and Asterion. Angry over his loss, Poseidon caused these Rivers to dry up (Paus. ii. 15. § 5, 22. § 5; Apollod. ii. 1. § 4.). Even with his brother and King, Zeus, Poseidon disputed ownership of the island of Aigina; with Dionysos that of Naxos (Plut. Sympos. ix. 6.).
Yet another new piece of information to me is Poseidon’s role in the story of the adultery of Ares and Aphrodite. When Hephaistos had caught the adulterers in the act in his net and had fetched to Gods to behold the sight, all the Gods laughed about it. Only Poseidon did not and beseeched Hephaistos to let Ares go, promising Ares would repay him whatever he asked as compensation. Later, however, Poseidon would charge Ares with murder for killing Halirrhotios, one of Poseidon’s sons. Ares had done this because Halirrhotios had sexually assaulted a daughter of Ares, Alkippe. The trial took place in Athens on what was later named the Areiopagos, where the Gods ultimately found Ares not guilty of murder and that he had just cause for killing Halirrhotios.
There are certainly far more relations that I have skipped over or not mentioned at all, but I hope these will do.