"Onix has a magnet in its brain. It acts as a compass so that this Pokémon does not lose direction while it is tunneling. As it grows older, its body becomes increasingly rounder and smoother."
While Onix may look like a simple rock-snake, it's actually one of the largest Pokemon around, capable of giving rides to numerous people, and making massive tunnels when it burrows into the ground. Onix is a staple of Rock-type trainers, including Brock, the first Gym Leader of Pokemon Red/Blue and Yellow. And the second friend Ash made on his journey. Third if you count Pikachu.
Onix's Basic Attacks are Rock Throw or Tackle, while its Special Attacks are Iron Head, Rock Slide, or Stone Edge.