Brock's first Pokémon was an Onix given to him by his father Flint when he was 10 years old. In A Family That Battles Together Stays Together, he gave Onix to his younger brother Forrest and prior to Grating Spaces!, Forrest got Onix to evolve into a Steelix.
Ash and friends encountered a giant Onix in To Master the Onixpected. It was on a rampage until Bruno managed to get a Sandslash from under the Rock Snake Pokémon's "skin." Bruno captured the Onix afterward.
The Crystal Onix featured a special Onix whose body is made out of glass crystal. Ash and his friends helped a glass sculptor named Mateo search for it because Mateo believed it would help inspire him to be better at his craft.
In Hello Pummelo, Drake of the Orange Crew owns an Onix and he used it to fight Ash's Squirtle in the second round in the Orange League Champion match. During the battle with Squirtle, Drake's Onix dug underground and emerged to use Bind on Squirtle, but Squirtle used Withdraw followed by a Hydro Pump, soaking Drake's Onix into submission. After drenching Drake's Onix, Squirtle defeated the Onix with a Skull Bash. It reappeared in a flashback in the following episode.
A "King Onix" was featured in Hooked on Onix. Team Rocket tricked King Onix into thinking Ash and his friends were going to destroy his kingdom. When Ash managed to convince King Onix that he and his friends weren't bad by defeating it in a battle, Team Rocket tried to steal the royal Rock Snake Pokémon, but their plan was foiled, as usual.
Roark's Onix was first seen in Shapes of Things to Come where it was used against Paul's Elekid and Chimchar. In A Gruff Act to Follow, it defeated Ash's Turtwig, followed by Ash's Pikachu but Pikachu managed to defeat it in a rematch in O'er the Rampardos we Watched.
Brock used an Onix in One Team, Two Team, Red Team, Blue Team! as part of the Pokémon Triathlon.
An Onix appeared in Mewtwo Strikes Back. Multiple Onix were among the Pokémon seen at Professor Oak's Laboratory in Showdown at the Po-ké Corral. An Onix, under the ownership of an unnamed Trainer, was treated by Nurse Joy in The Joy of Pokémon. An Onix belonging to the natives of the Golden Island, appeared in Meowth Rules!. Multiple Onix also appeared in The Power of One. An Onix was seen in a fantasy in The Rivalry Revival. An Onix, under the ownership of a trainer, appeared in Celebi: Voice of the Forest. An Onix appeared in the opening of Destiny Deoxys. Multiple Onix appeared in Lucario and the Mystery of Mew as part of the feuding armies. An Onix made a brief cameo in Pokémon Mystery Dungeon: Explorers of Sky - Beyond Time & Darkness. An Onix appeared in Steeling Peace of Mind!, and again in Promoting Healthy Tangrowth!. An Onix appeared in A Rivalry to Gible On!. An Onix appeared in Bucking the Treasure Trend! where it attacked Team Rocket. An Onix appeared in the opening scenes of Zoroark: Master of Illusions.