He made several sea creatures including the horse
To impress Demeter since Demeter asked him to create the most beautiful animal, so that she would start liking him. But after a lot of mistakes(zebra, giraffe, camel) Poseidon finally made the horse but he no longer liked Demeter anymore.
Demeter asked Poseidon to help her find her daughter Persephone, who had been taken to the Underworld by Hades. Poseidon revealed that Hades was responsible and guided Demeter to him.
In Greek Mythology, Zeus was holding a contest to decide which god, Athena or Poseidon, would gain the love from the people of the Acropolis. Whichever created the most useful tool would win. Poseidon created the horse, but Athena won with the olive tree.
Once Posideon made a god very very verryyy angry. He was dissapointed in him, and the only way Posideon could impress him again was to make a creature like no other! Therefore here came the horse. The god was so pleased, and now that... is Posideon's horse tale.
Amphitrite married as Poseidon asked her to be his wife.
Medusa never asked to be a gorgon. Athena turned her into one when she caught her messing around with Poseidon in her temple. Her devious nature and killing streak is probably fueled by her frustration with not being a beautiful young maiden anymore.
Cyclops asked Poseiden to add to Odysseus' fate.
Polyphemus asked his father Poseidon to take justice on Odysseus.
Hades loved her and asked Zeus for her hand in marriage - which Zeus granted - but did not inform Demeter or Persephone.
Zeus asked Iris to tell Demeter to stop being angry over Perspherone living with Hades.
The 3 main gods are Zeus, Hades, and Poseidon. The gods asked as part of the Aphrodite test (Poseidon's realm) are Ares, Hermes, Aphrodite, Artemis, and Hestia.
Yes, she became a maiden after Poseidon and Apollo asked for her hansd in marriage and she swore to never marry.