This giant Onix was first seen rampaging wildly around Mt. Hideaway. Ash and his friends were on the Onix's trail and got chased. The chased ended after they hid behind a rock. Later, it rose from the ground and encountered Team Rocket. Team Rocket fired a net at Onix which got it angry. Team Rocket retreated to a cave and Onix tackled the cave.
Ash, Misty, and Brock sent out Squirtle, Staryu, and Onix respectively to battle the giant Onix but didn't succeed. Ash fell from the cliff and Onix was about to attack him, but Bruno appeared and saved Ash from the attack. He was ready to catch Onix. When Ash and the others asked what Pokémon he was going to use, Bruno shocked them by explaining that he wasn't going to use any Pokémon.
Bruno jumped down to find out why Onix was violent. Bruno avoided the hits thrown by Onix. After a while, Bruno found the reason why Onix was on a rampage: a Sandslash had been stuck between two of its boulders, which was hurting it. After Bruno removed the Sandslash, Onix became very thankful for him and accepted to join his team, so Bruno threw a Poké Ball at Onix and captured it. It hasn't been seen since.
Personality and characteristics
Onix was usually a friendly Pokémon but because of a Sandslash being stuck inside it, it caused Onix to wither in pain, going on a rampage. Throughout the episode Onix appeared in, it had very little trouble dealing with Team Rocket's bazooka and Pokémon with the type-advantage.
In Can You Diglett?, Bruno used his Onix to battle Red during their battle on Mt. Moon. Despite a type disadvantage against Gyara, it was able to hold its own and deftly avoid super-effective hits from its Water-type attacks. Afterwards, it used its Bind to keep the Atrocious Pokémon from attacking at all, only to take damage from the latter's Bite. It then dug underground to launch a surprise attack, and this time faced off against Saur. However, before that fight could conclude, it was interrupted by a pack of Diglett crossing, and later Bruno's Elite Four comrades Lorelei and Agatha.