Brock's Steelix (Japanese: タケシのハガネールTakeshi's Haganeil) was the first Pokémon acquired by Brock in the anime, and is his main battling Pokémon in both the anime and games.
Its voice actor in both Japanese and English is 石塚運昇 Unshō Ishizuka.
Brock received it as an Onix as a present from his father, Flint, during his tenth birthday, as seen in a flashback in A Wailord of a Time!.
Like most of Brock's Pokémon, Onix received very little screentime in comparison to the Pokémon of the other main characters. This may have been due to its massive size, although it did have an impressive battle against Iron-Masked Marauder's Tyranitar in Celebi: Voice of the Forest. Its size was also used in several ways, such as a staircase in Pokémon Shipwreck. Like most of Brock's pre-Hoenn Pokémon, Onix does not have a distinct personality.
Brock ended up giving Onix and two of his other Pokémon to his eldest sibling, Forrest, who took on the role of Gym Leader at Pewter City. When Brock returned to the gym in Grating Spaces, Onix had evolved into a Steelix under Forrest's care.
In Pokémon Adventures, Red challenges Brock's Onix during Onix is On! in a gym battle to gain the Boulder Badge. Red uses Pika to battle, but it is still angry with Red for capturing it. Onix first uses Rock Throw at Pikachu causing massive damage before proceeding to use it's body like a torpedo to hit Pika with Skull Bash, however Red pushes Pika out of the way. Trying again, Pikachu uses a powerful electric attack, on Onix breaking apart the rocks that its body is composed off.
In Pokémon Zensho, Satoshi battles Brock's Onix in Nibi City. Satoshi uses his Charmander in the battle, and tells it to chop the horn-like part of Onix's head off. Onix is then defeated, and Satoshi wins the battle.
In the TCG
Brock's Steelix was featured in the TCG multiple times. The following is a list of cards named Brock's Onix.
Onix took the longest of all Pokémon in the anime to evolve after its introduction, taking at least 403 episodes (in A Real Cleffa-Hanger, Brock went to the Gym), and at most 446 episodes (in Grating Spaces, Brock was surprised to see it) to become a Steelix.