This Horsea is a Water-type Pokémon owned by Misty.
In Tentacool & Tentacruel, Horsea is found injured near a coast by Misty. Horsea tries warning Misty and her friends about the Tentacool and Tentacruel terrorizing the people of Hutber Port, but they don't understand. However, the gang finds out what it meant after witnessing for themselves. Horsea takes a liking to Misty and allows itself to be captured.
In The Misty Mermaid, Horsea begin 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.
In Cerulean Blues, Horsea is seen again when Misty visits the gym. It helps push Misty afloat when she begin drowning in the water because of a Gyarados.