Horsea first appeared in Tentacool & Tentacruel. Brock, Ash, and Misty were trying to return to the mainland when Misty noticed an injured Horsea near the coast at Porta Vista. The little seahorse squirted out a warning in ink that Ash and his friends didn't understand. Later, the group learned that a woman named Nastina was trying to build an underwater resort in the local reefs, and that her workers were being attacked by many Tentacool. A strange chain of events lead to the community being attacked by a gigantic Tentacruel, who was angry at the humans demolishing the reef. Horsea and Misty's pleading saved the city, and Horsea then joined Misty's team.
Horsea next appeared in The Battling Eevee Brothers, swimming in a fountain. Horsea is later captured by Team Rocket alongside the rest of the gang's Pokémon. Using its ability to spray ink Horsea leaves a trail on the ground so everyone will be able to follow them, which Ash's Pidgeotto notices. Horsea later sprays ink on Jessie to distract her when the gang shows up to rescue them.
Much later, in The Misty Mermaid, Horsea was feeling ill because Misty wasn't letting it out enough to get any exercise. With no ocean nearby, Misty decided to take it home to Cerulean City, where it could play and swim in her Gym's many aquariums. After participating in a water ballet, Misty left Horsea and her Starmie in the care of her sisters.
As shown in its debut, Horsea prefers to use words before battling head on. Horsea, like Goldeen, has trouble functioning on land, so Misty didn't use it much in her battles. However, when it is used, it proves to be very resourceful - using ink to draw pictures or leave trails. It, like the rest of its species, prefers large bodies of water to swim in, hence, it grew sad and ill when it was forced to swim in small puddles of water, and Misty left it in the Cerulean Gym, so that it could swim to its heart's content in the Gym pool.
Misty's Horsea appeared in Cerulean City where it was seen in her Gym.
In the TCG
Horsea was featured in the TCG as one of Misty's Pokémon. The following is a list of cards named Misty's Horsea.
Misty's Horsea Cards listed with a blue background are only legal to use in the current Expanded format. Cards listed with a silver background are legal to use in both the current Standard and Expanded formats.