Onix is a Pokémon composed of a giant chain of gray boulders that become smaller towards the tail. Its length makes it the tallest Rock-type Pokémon. It has black eyes and a rounded snout. On top of its head is a rocky spine. As it grows older, it becomes more rounded and smoother, eventually becoming similar to black diamonds. Inside its brain is a magnet that serves as an internal compass, guiding Onix wherever they need to go and prevents them from getting lost while digging.

Onix tunnels under the ground at over 50 miles (80 kilometers) per hour, which causes tremors and a terrifying roar that echoes a long way. Hard objects encountered while tunneling are absorbed into its body, which makes it very sturdy. It also consumes the boulders for food. The massive, winding tunnels it leaves behind are used as homes by Diglett. As seen in the Pokémon Stadium series, Onix is capable of rotating its head, or any part of its body, in a full 360 degrees. It is a very aggressive Pokémon that will constantly attack humans and other Pokémon when in pain, as seen in the anime. As seen in the Pokémon Adventures manga, Onix can be separated from its body segments in battle and be fine later on, either regenerating or reattaching to its body segments. It is said that an Onix that lives for about 100 years will evolve into a Steelix after its composition becomes more diamond-like. It can be found in mountains and caves.

Onix evolves into Steelix.

(Source: Bulbapedia)