Onix is a Rock Snake Pokémon. Its serpentine body is made up of large boulders and little pebbles. It has a long horn atop its head and small eyes.
Onix can have the abilities Rock Head or Sturdy. Rock Head prevents Onix from recoiling, while Sturdy gives Onix immunity to OHKO moves . When it is burrowing underground, it can cause rumbles and earthquakes. It can dig at speeds up to 50 MPH. It has a compass in its brain that can tell Onix what direction it is facing.
Brock received an Onix from his father, Flint, on his 10th birthday, as seen in a flashback in Having a Wailord of a Time . Onix was Brock's 1st Pokémon and has consistently been his most powerful Pokemon as well.
The Crystal Onix's first and only appearance is in the anime episode The Crystal Onix. Ash and his friends first heard of it through a note they found in a bottle floating in the ocean. The note was written by a little girl named Marissa. She wanted to find the Crystal Onix to help her brother, Mateo, gain inspiration to make glass Pokémon statues. Mateo told a story of how his grandfather discovered the Crystal Onix and then was able to make extraordinary sculptures afterwards. One of those sculptures included the Crystal Onix itself. In the Best Wishes in the episode, Expedition to Onix Island!, a shiny Onix is the leader of several normal Onix in the Onix Island. When Ash and his group are chased by most of the normal Onix, Meloetta did her best to sing her song to calm them down but its no effect until a shiny Onix loudly roars to stop them which causes all of the normal Onix leave and allowing Ash and the group to see its shiny Onix before leaving back to Cynthia's resthouse.