Poseidon

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

Hercules (film)
In the film he was present for the baby 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 major part of the plot.