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 long rock spikes coming out of them on either side. It has a massive head mainly made up of its large, wide jaw with sets of large square teeth inside. Along its 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. Its head has two long ridges going from the eyes up to the top of its head. The tip of its tail is conical with a blunt tip.
As Mega Steelix, its body becomes longer, almost doubles its mass, and gains more features. Mega Steelix's eyes turn blue and its lower jaw becomes more heavily armored with a round, curved shape. Additionally, a third pair of ridges appear on its bottom jaw, as well as two sharp, flat structures consisting of three conjoined projections on each side. 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; these scraps react 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 up to six-tenths of a mile underground, even further than its pre-evolved form Onix, and it can be found in mountains or caves. Steelix can dig even towards the world's core. It easily chews through boulders with its powerful jaws, and its eyes are capable of seeing in the dark. Exposure to underground heat and pressures alongside its iron-rich soil diet has made its body harder than diamonds. Steelix's evolution is said to have been the result of an Onix's composition becoming more diamond-like after living for about 100 years.
In Dealing With Defensive Types!, Byron used a Steelix in his Gymbattle against Ash. It put up a tough fight, defeating Buizel despite its type advantage, and then fought Chimchar. Despite its exhaustion from its earlier battle, Chimchar was able to defeat Steelix after a prolonged battle.
In Pokémon Adventures, Jasmine used a Steelix disguised as an Onix in the Gym Leader's Tournament of the opening ceremonies in the Pokémon League. Brock's Onix tightened its grip on the hidden steel Pokémon, but against the unusual hardiness of its evolution, it only hurt itself and was eventually defeated after Steelix's stone "hide" scraped off and it was exposed. Jasmine explains that because it was a newly discovered species only so far uncovered by Crystal's capture, she tried to hide its identity, so to ensure that it wouldn't get the audience scared.
A combination of Steel- and Ground-types, Steelix are the longest Pokémon yet to be discovered, reaching over 30 feet in length. Since they live so deep under the surface of the earth, the intense pressure has made their bodies stronger than diamonds. Evolved from Onix, Steelix is every bit as intimidating as its relative.
NA:With a body harder than diamond, this Pokemon is well suited to digging, using its powerful jaws to tunnel around. Steelix lives deep underground - even deeper than its pre-evolution, Onix. Was it pride that made Steelix prove itself able to go farther? It's hard to say. We tried to dig around for clues, but we got tired after only 10 feet...
PAL:Being harder than diamond, Steelix can crush any rock with its jaw. It can also see in pitch darkness. Its territory lies deeper than its pre-evolution, Onix, a full 1000m below the surface. Does it consider its Onix brethren to be "below" it, even though they're higher up? It's hard to say. We may have to dig a little deeper into the matter.
Mega Steelix is the tallest Steel- and Ground-type Pokémon.
Steelix can be seen as a parallel to Scizor. They both have pre-evolved forms from Generation I, and both evolve via trading with a Metal Coat in which both change a typing to the Steel type. They both also gained a Mega Evolved form in Generation VI.
Steelix, like Onix, is most likely based on a combination of a snake and earthworm. It may also be based on the wyrm, a type of dragon that has been presented as limbless and burrowing. Its shiny coloration is likely based on the fact that gold is a very rare metal. Mega Steelix's crystalline body may be a reference to graphite turning into diamonds from compression.
Steelix is a combination of steel and Onix, which itself likely came from the onyx stone. It may also refer to helix (with regards to its serpentine body).
Haganeil may be a combination of 鋼 hagane (steel) and nail or tail.