Horsea is a Generation I Pokemon, which means it was introduced in the original Pokemon Red and Blue games. It is not available in the remakes, Pokemon FireRed and LeafGreen.
You can find Horsea in Pokemon Emerald, Diamond/Pearl/Platinum, Black/White/Black 2/White 2, X/Y, Omega Ruby/Alpha Sapphire, Sun/Moon/Ultra Sun/Ultra Moon, Let's Go Pikachu/Eevee, Sword/Shield, and Scarlet/Violet.