He first appeared as an Onix in Showdown in Pewter City, where Brock used it to battle Ash's Pikachu. Because he was part Ground-type, he was unaffected by Pikachu's electric attacks. However, after he was dowsed with water, Pikachu was able to affect him with his electric attacks, and ultimately knock it out.
When Brock joins with Ash and Misty from their travels, Onix is also used on some occasions, such as stopping Team Rocket from stealing the large Moon crater using Dig, entering a Pokémon-athon race but he ends up disqualified because of his dislike of water, and Onix cries towards its trainer.
In Johto from the episode Sick Daze, Brock ends up getting sick and he forces Ash and Misty to deal with his note chores such as food and polishing rock Pokémon. After realizing how hard he is to polish them, Ash decides to use Totodile to wash them as Misty scolds him that rock Pokemon hates water when Onix was seen panicking and dodges to avoid a Water Gun. Later, Onix and the other Pokémon are captured by Team Rocket before Brock's Golbat manages to slice the Meowth Balloon with his 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 at the Dragon Holy Land, Brock releases both Onix and Geodude as the latter to pull off his Rock Throw in order to stop the fire while Geodude rescues other Pokémon from being trapped.
Advanced Generation series
In Pokémon Chronicles from the episode, A Family that Battles Together Stays Together! While still an Onix, Brock uses him to fight his ditzy mother, Lola who is responsible for redecorating 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 he uses Dig to remove the water from the field and attacks Mantine underneath. He 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 him to his younger brother, Forrest, the next Gym Leader of Pewter City Gym to take care of him 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 him and became mostly pleased to his younger brother, Forrest for taking care of his Pokémon and letting his Onix to evolve. He also uses him 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.
Diamond and Pearl series
During Paul's first Gym battle against Roark who also owns both Geodude and Onix. Brock mentions to Dawn that he also owns an Onix who has now evolved into a Steelix and remarking of how Roark raises his own Onix. This also happens in Dealing with Defensive Types! when Brock was surprised to see Byron's Steelix also gains a Screech ability when it uses to lower more of Ash's Buizel's defense before it quickly defeats it. He also remarks that this ability is also shared from Roark's Onix knowing of their family bloodline while raising both of their Pokémon.
Sun & Moon series
In the Sun & Moon series after both Brock and Misty befriended five of Ash's friends from the Alola region, Brock releases Steelix during his Gym battle against Kiawe's Turtonator at the Cerulean City Gym. Steelix is able to withstand Turtonator's Flamethrower despite it being super effective. Like Geodude, Steelix uses its newly learned Gyro Ball to disperse the flames followed by Bind until it broke free due to its Shell Trap. When Turtonator unleashes its Dragon Tail, Steelix manages to use Dig followed by its tail Slam as a combo hit. When Brock removes his vest and shirt to reveal the Key Stone from his necklace, he rips his necklace to activate his Key Stone and he Mega Evolves his Steelix into Mega Steelix, much to Kiawe's horrible shock. Despite Kiawe's efforts to defeat it using Turtonator's Inferno Overdrive, Mega Steelix withstands the Z-Move and ultimately defeats it with one powerful Stone Edge when the former tries to disperse its powerful attack with Dragon Tail. Despite Kiawe's loss, he was still given a Boulder Badge along with Sophocles by Brock.
- Steelix is the first Pokémon to evolve off-screen.
- Steelix is the first Pokémon belonging to a main character to use Mega Evolution.
- ^ AG034: Having a Wailord of a Time
- ^ IL005: Showdown in Pewter City
- ^ IL006: Clefairy and the Moon Stone
- ^ IL033: The Flame Pokémon-athon!
- ^ IL059: Volcanic Panic
- ^ JE077: Sick Daze
- ^ JE137: Great Bowls of Fire!
- ^ A Family That Battles Together Stays Together!
- ^ AG178: Grating Spaces!
- ^ DP015: Shapes of Things to Come!
- ^ DP108: Dealing With Defensive Types!
- ^ SM043: When Regions Collide!