Evolving Onix into Steelix is a process that can be completed in the Pokmon video games. Onix is a Rock/Ground type Pokmon that evolves into Steelix, a Steel/Ground type Pokmon, when it is traded while holding a Metal Coat.
Steelix is a powerful Pokmon that can be a valuable asset to any team. It has high Defense and Special Defense, making it a great tank. It also has a good Attack stat, making it a decent attacker as well. Steelix is also immune to Poison-type moves, which can be very helpful in certain battles.
