Steelix, the Iron Snake Pokémon. Steelix is the evolved form of Onix. It has been compressed underground, making its body harder than diamonds.
– Ash’s Pokedex
Steelix is Onix’s evolution in Gen II, when the Steel type was “discovered”. It’s one of the few Pokemon to change its primary type when evolving. Up until Gen V, no other Pokemon has the same type combination as Steelix. Excadrill is Ground/Steel, while Steelix is Steel/Ground. Steelix still has the same Fighting and Ground weaknesses as Onix, though it is no longer weak to Grass and only normally weak to Water. However, it gained a weakness to Fire.
It looks rather mean, and quite top-heavy. A lot of Gym Leaders and Elite Four members have Steelix as part of their team, including Flint of the Sinnoh Elite 4, who despite being a Fire type trainer has a Steelix (who is weak to Fire).
In the anime, Brock’s Onix evolved into Steelix after it was left with Brock’s little brother who took over the gym while Brock was travelling with Ash.
Stat-wise, Steelix has a LOT of Defense (base 200 Defense). Its Attack is decent, but everything else is rather low, especially is base 30 Speed.