Onix

Onix is a large rock snake Pokemon. It is a dual Rock/Ground type Pokemon, which means that Electric attacks don't work on it. Onix is known very well for being one of the Pokemon owned by Gym Leader Brock. It had been very iconic ever since.

At first, Onix didn't have an evolutionary line until Generation II when it is discovered that it can evolve into Steelix, a dual Rock/Steel type Pokemon, when it's traded away while holding a Metal Coat.