You can only get an "onix" in Pokemon sapphire if you have a Pokemon version that has an onix then use a linking cable with other GBA to trade it and then you'll have an onix but "onix's" data can't be stored in your pokedex.
NOTE:You can only trade with a Pokemon version that have the same region as your Sapphire has;HOENN REGION.
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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.
Need a metal coat and trade like a golem I like Pokemon so much that i almost played every Pokemon game
If you mean,, what onix evolves into,, it's steelix,, but the is no Pokemon that evolves into onix
onix is the strongest
Onix doesn't evolve from anything. It evolves into Steelix, but Onix is the base pokemon for that evolution chain. :)