Onix was given to Brock by his father, Flint, on his 10th birthday.
It first appeared as an Onix in Showdown in Pewter City, where Brock used it to battle Ash's Pikachu. Because it was part ground-type, it was unaffected by Pikachu's electric attacks. However, after it was dowsed with water, Pikachu was able to affect it with its electric attacks, and ultimately knock it out.
When Brock joins with Ash and Misty from their travels, Onix is also used in some occasions, such as a Pokémon race; it ends up disqualified because of its dislike of water, and Onix cries towards its trainer.
Later in Volcanic Panic! Onix and Geodude are aiding Ash's Charizard and Pikachu and Blaine's Magmar to stop the lava flow that destroys Blaine's gym.
In Johto from the episode Sick Daze, Brock ends up getting sick and it forces Ash and Misty to deal with his note chores such as food and polishing rock Pokemon. After realizing how hard to polish them, Ash decides to use Totodile to washed them as Misty scolds him that rock Pokemon hates water which Onix was seen panicking and dodges to avoid its Water Gun. Later, Onix and the other Pokemon are captured by Team Rocket before Brock's Golbat manages to slice the Meowth Balloon with its Wing Attack. Onix uses Rock Throw on Team Rocket before being blasted off by Pikachu's Thunder.
In Great Bowls of Fire! during Dragonite's rampage, Brock gets Onix to pull off its Rock Throw in order to stop the fire.
Advanced Generation series
In Pokemon Chronicles from the episode, A Family that Battles Together Stays Together! While still an Onix, Brock uses it to fight his ditzy mother, Lola who is responsible for redecorate the Pewter Gym into an aquarium filled with her water Pokémon in order to put the gym back to its former glory. Onix fights Lola's Mantine and it uses Dig to remove the water from the field and attacks Mantine underneath. It got hit from Mantine's BubbleBeam before Onix grips it with Bind and defeats it with Slam when it attacks continuously from its BubbleBeam. Later, Brock lent it to his younger brother, Forrest, the next Gym Leader of Pewter City Gym to take care of it along with his other Pokémon before he left to Hoenn to meet up with Ash, May and Max.
When Brock returned to Pewter City in the episode Grating Spaces!, he discovered that his Onix had evolved into Steelix as he grips his trainer as a reunion and Brock recognizes it and became mostly pleased to his younger brother, Forrest for taking care of his Pokémon and letting his Onix to evolve. He also uses it to team up with Ash's Donphan against the gym remodelers which is Jessie and James in disguise and using two of Butch and Cassidy's Pokémon, Charizard and Aggron against them. After Team Rocket is revealed to be responsible for remodeling the gym and Meowth's attempt on stealing the Gym's Pokémon, Ash, Brock and Forrest manage to blast them off with a combination of Pikachu's Thunderbolt, Donphan's Hyper Beam, Steelix's Dragon Breath and Geodude's Sandstorm. After the incident, Brock continues his travel with Ash, May and Max and gives farewell to his family and his Pokémon.
|Tackle||Showdown in Pewter City|
|Bind +||Showdown in Pewter City|
|Dig||Clefairy and the Moon Stone|
|Rock Throw||Who Gets To Keep Togepi?|
|Sandstorm||Entei at Your Own Risk|
|Slam||A Family That Battles Together Stays Together!|
|Iron Tail +||Grating Spaces!|
|Dragon Breath +||Grating Spaces!|
|+ indicates this Pokémon used this move recently.*|
- indicates this Pokémon normally can't use this move.
Unshō Ishizuka (Japanese and English)
At Pewter Gym/Home
Caught for someone else