It evolves from Onix when traded holding a Metal Coat. It can Mega Evolve into Mega Steelix using the Steelixite.
Steelix is a long, serpentine Pokémon with a silvery-gray body made up of rock-like sections, which can possess chips and scrapes from fights and tunneling. Three of those sections have massive rock spikes coming out of them. It has a massive head mainly made up of its large, wide jaw with sets of large square teeth inside. Along it bottom jaw are ridges: there are two pairs of ridges on a male Steelix, while there is only one pair on a female. Several small, square lumps cover the underside of its lower jaw. Its eye sockets are slightly deep set, and the eyes themselves are red with white sclera. Its head has two long ridges going from the eyes up to the top of its head. Its tail ends in a long sharp point, but with a blunt tip.
As Mega Steelix, its body becomes longer, almost doubles its mass, and gains more features. Mega Steelix's eyes attain a blue color and its lower jaw becomes more heavily armored, gaining a round, curved shape. Additionally, a third pair of ridges appears on its bottom jaw, and two sharp, flat structures comprised of three conjoined points protrude from the front of its lower jaw. The gray, rock-like spikes and its tail point have transformed into prism-like crystalline spikes; the segments from which the spikes protrude are much larger as well. In-between the head and individual spiked segments are smaller units, now adorned by a black spot with a blue hexagon on each side. Several metallic-looking scraps constantly rotate around its neck, vaguely resembling hands; these scraps act to Mega Steelix's movements and change position when attacking. The cells within its body, crystallized by the energy produced from Mega Evolution, are stronger than any mineral and able to withstand any temperature. It moves incredibly slowly, but its strengthened body is also impervious to damage, and it lashes out at foes without flinching.
Steelix burrows underground and can be found in mountains or caves. It lives deep underground, and has been found as deep as six tenths of mile. It is capable of chewing through boulders with its powerful jaws, and its eyes are capable of seeing in the dark. Its body is made harder than diamond by the iron it ingests when it eats soil. It's further strengthened by the pressure and heat underground.
- Mega Steelix is tied with Mega Aggron and Shuckle for the highest Defense stat of all Pokémon.
- This means Mega Steelix has the highest Defense base stat and the highest single base stat of all Ground-type Pokémon, and is tied with Mega Aggron for highest Defense base stat and highest single base stat of all Steel-type Pokémon.
- Steelix is shown floating above the ground in all console games, rather than, like its pre-evolved form, resting on the ground.
- All Steelix in the anime have used the move Iron Tail (excluding cameos).
- Steelix is the only Pokémon so far in the Pokémon Ranger games that can hurt the Capture Styler simply by touching the line.
- Mega Steelix is the tallest Steel-type and Ground-type Pokémon.