Onix is Brock’s signature partner in the Pokémon TV series.
Brock is the former Pewter City Gym Leader and specializes in Rock-type Pokémon. His Onix is one of his most recognizable team members and is frequently used in battles during the original series.
Onix is a Rock- and Ground-type Pokémon shaped like a giant segmented stone serpent. In the games and later stories, it can evolve into Steelix when traded while holding a Metal Coat, but Brock’s original Onix is not the same Pokémon as Steelix.
Brock also uses Pokémon such as Geodude and Vulpix, so Onix is not his only partner. However, its size, distinctive design, and repeated appearances make it the clearest signature Pokémon associated with him in the anime. Brock travels with Ash after losing his Pewter Gym battle and remains a major companion across several series.