Give it a metal coat and trade it or go to Iron Island and you can find wild Steelix there. To get to Iron Island go to Canalave city and talk to the dude by the boat and he will ask you if you want to sail or something like that, and you can choose to sail to Iron Island.
no onix evolves in pokemon ruby , saphire and emerald but i do not know what level it evolves , it might evolve by trade
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you can evolve him into steelix otherwise i think not. heres a drawing of an ass ( Y )
In Pokemon Yellow Onix does not evolve, it is a one stage Pokemon.
In order to evolve Onix in Pokemon Crater you must beat Cynthia. After beating Cynthia she will give you the mental coat to evolve Onix.
Onix doesn't evolve at a level. You have to give it the item 'Steel Coat' and trade it to make it evolve into Steelix. Or just catch a steelix at Victory Road using dusk balls
Onix doesn't evolve from anything. It evolves into Steelix, but Onix is the base pokemon for that evolution chain. :)