Poseidon

Poseidon is the Olympian God of the seas and older brother of Zeus and Hades. Despite his small role in the film he played a larger role in the animated series. He is voiced by Jason Alexander.

Hercules (film)
In the film he was present for the party celebrating the arrival of Zeus and Hera's newborn child Hercules. Years later Poseidon and his fellow Gods are imprisoned by Hades and the Titans, luckily he and the other Gods were freed by an adult Hercules.

Hercules (animated series)
In the animated series he played a much more larger role where he would often appear in either a cameo or as a major part of the plot.

Hercules and the River Styx
In this episode Poseidon is visited by his brother Hades who wants to trick him into using his powers to move the River Styx (which is part of Hades' domain) into Greece (which is part of Zeus' domain), this will instead turn Greece into part of Hades domain and therefore make him ruler of Olympus. He then tricks Poseidon by telling him that when he becomes ruler he will give Poseidon the city of Athens.

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, overhearing what his cousin thought of him, Triton has stolen Poseidon's trident and accidentally unleashed a sea monster. The two cousins team up to defeat the monster, and Poseidon arrives to take back his trident, not punishing Triton thanks to Amphitrite intervening.

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), wife of Poseidon, has taken a special interest in Icarus, and helps he and his father Daedalus complete 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.