Onix does not have a pre-evolution form currently as of Pokemon Omega Ruby and Alpha Sapphire and has not been announced for the Pokemon Sun and Moon games.
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If you mean,, what onix evolves into,, it's steelix,, but the is no Pokemon that evolves into onix
diglett evolves into dugtrio, onix is its on Pokemon that evolves into steelix using metal coat.
Level: Onix Evolves By Trading.
Onix doesn't evolve from anything. It evolves into Steelix, but Onix is the base pokemon for that evolution chain. :)
These Pokemons evolve when the Shiny Stone is used: Togetic, Roselia, Luxio and onix evolve. Togetic evolves into Togekiss, Roselia evolves into Roserade, and onix evolves into steelix. Also, in Pokemon Black you can evolve Minnicino.