Brock's Steelix is one of Brock's Pokémon his father caught as an Onix in Kanto. It evolved into Steelix (not shown) and was discovered in Battle Frontier series. Steelix was given to Brock's brother, Forrest, the next Pewter CityGym Leader. It still resides with Forrest in the Pewter Gym.
It is voiced by Unshō Ishizuka who plays the Narrator in the Japanese version in both the Japanese and English-language anime.