you can evolve him into steelix otherwise i think not. heres a drawing of an ass ( Y )
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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. :)
To evolve onix you must give it a metal coat then trade it and it will evolve