Poseidon

Poseidon is the Olympian God of the seas and the brother of Zeus and Hades. Despite his small role in the film, he plays a larger role in the animated series.

Hercules
In the film, he is present for the party celebrating the arrival of Zeus and Hera's newborn child, Hercules. Eighteen years later, Poseidon and his fellow gods are imprisoned by Hades and the Titans. He and the other gods are freed by Hercules.

Hercules: The Animated Series
In the animated series, Poseidon plays a much larger role than in the movie, either by acting as a major part in the plot or by appearing in a cameo.

In "Hercules and the River Styx", Poseidon is visited by his youngest brother, Hades, who tries to trick him into using his powers to move the River Styx (a part of Hades' domain) into Greece (a part of Zeus' domain). Such a ploy would place Greece in Hades' control, therefore making him the ruler of Olympus. In exchange, he promises to give Poseidon the city of Athens.

In "Hercules and the Son of Poseidon", Poseidon introduces Hercules to his son, Triton (Chris Elliott), a sea prince who wants to go to Prometheus Academy with him. It turns out Triton is somewhat of a nerd, wanting to be like his cousin; he has considerable acrobatic skills but terrible balance and timing, much like Hercules himself, causing him to be a constant source of humiliation. Icarus and Cassandra remind Hercules that he and his cousin are not so different, so he tries to apologize. Unfortunately, having overheard what his cousin thought of him, Triton steals Poseidon's trident and accidentally unleashes a sea monster. The two cousins team up to defeat the monster, and Poseidon arrives to retrieve his trident, not punishing Triton due to Amphitrite intervening.

In "Hercules and the Poseidon's Cup Adventure", Poseidon, tired of being forgotten, holds a boating race in his honor with Adonis competing for Thrace against Hercules and Icarus. However, when their star rower Hylas (Rocky Carroll) is injured, the King of Thrace hires Hercules instead, who abandons Icarus. Fortunately, Amphitrite (Leslie Mann), the wife of Poseidon, has taken a special interest in Icarus and helps both him and his father, Daedalus, in completing their boat. At a regala held the night before the race, Hercules declares himself bigger than Poseidon, which causes the sea god to unleash Charybdis, a giant maelstrom sea creature, during the race, which causes both his and Icarus' ships to nearly sink. Rescuing first his friend and then Adonis, Hercules looks on as Icarus wins the Poseidon Cup, being the only one to have truly earned it.

Once Upon a Time
Poseidon reigns as the sea king of his underwater kingdom. With his unnamed wife, he has a daughter, Ursula, who was named after the sea goddess of the same name.

His wife and daughter, both renowned for their singing voices, gave him much happiness. However, when his wife is killed by a pirate, Poseidon comes to hate humans and has Ursula lure sailors to their deaths with her enchanting voice. During one such excursion, she allows the crew aboard the Jolly Roger to live and incurs her father's disappointment. Longing to honour her mother's memory, Ursula doesn't wish to use her singing talent to harm others and insists all humans can't bad, but Poseidon commands her to obey him as long as she lives in his kingdom. Upset at his demands, she steals a magic bracelet from his vault to gain human legs and disappears with plans of going to Glowerhaven to become a singer. Once Poseidon discovers Hook, captain of the Jolly Roger, has agreed to take Ursula there, he offers squid ink, which the pirate can use to give retribution to the Dark One. In return for this much needed item, the sea king enchants a shell and asks him to trap his daughter's voice within it so she won't leave the kingdom for Glowerhaven. Instead, Hook tells Ursula about the deal, and she procures the ink from her father's vault for him. Poseidon accosts them, and as Ursula sides with Hook, he takes away the ink to prove the pirate only cares about his vengeance. As expected, Hook is furious that his only hope of getting revenge is gone, and he gets back at Poseidon by absorbing Ursula's singing voice so the sea king can never sink another ship again. Under Hook's threats, Poseidon is forced to retreat without regaining the shell. Later, Ursula uses her father's trident to transform her mermaid fin into tentacles to become more powerful than him.

Years later and with Ariel's help, Poseidon travels to Storybrooke to meet with Ursula, who is now a grown woman. As the person who originally enchanted the shell, only he can restore Ursula's voice, which she desires to regain. During their reunion, Poseidon apologizes to his daughter for using her gift for horrible deeds and expresses regret for not cherishing her talent as a happy reminder of his late wife. After restoring her voice, he turns to leave, but a moved Ursula wishes to reconnect and decides to go home with him.

Powers and abilities
Poseidon is a very powerful god and equal to Zeus in power.
 * As an Olympian god, Poseidon possesses natural powers and abilities such as immortality, omnipresence, omniscience, superhuman strength, metamorphosis, and teleportation
 * As the god of the sea, Poseidon possesses the abilities to breath underwater, to communicate with sea creatures, and to control the sea with his Trident. Being the lord of the sea, Poseidon has authority over it and its creatures.

Trivia

 * Poseidon's Roman counterpart is named Neptune, which is also the name of the outermost and fourth-largest planet in the Solar System.
 * In Greek Mythology, Poseidon was typically characterized as a self-centered, angry and vengeful god, who would often use his powers to terrorize sailors, Port towns and civillization within reach. The Disney version, while still self-centered, is considerably more benign, and often characterized as a slacker.