Onix
The Rock Snake Pokémon
Type: Rock/Ground
Official Registration #: 96
Entry: A giant serpentine pokémon native to the cave systems of Kanto and Johto. As it grows, onix’s stone body hardens until its skin resembles diamond, which protects it as it burrows at high speeds in search of sustenance … all of which is not an extended euphemism, contrary to the opinions of the Sinnohan storage system administrator.
Steelix
The Iron Snake Pokémon
Type: Steel/Ground
Official Registration #: 208
Entry: The evolved form of onix, most easily via trading if the subject has had a metal coat applied to it. As this snake-like pokémon ages, it burrows deeper into the ground, where the pressure of the earth itself and the heat from the planet’s core tempers its steel skin. While tamed steelix may have this coat artificially applied to it before trading, it is possible for steelix to evolve from wild onix. Using this process, steelix initially have a rocky hide but form their coats from the minerals it consumes as it burrows. A sheath begins to form on its face first (as this is the first thing that burrows into the ground) but gradually slides backwards until it coats the entire length of the specimen. Friction compacts and hardens this coat until the aforementioned process of tempering (via pressure from the earth and heat from the planetary core) may occur. And no, this is still not an extended euphemism, Bebe.
The Pokédex According to Bill 