Well, if you are talking about it evolving I know the answer.
If you don't want to evolve then press B on the Ds. But once it stops the next level it will evolve again.
I hope my answer helped!!!
The Pokemon will stop evolving if you give it a everstone.
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There is 2 ways to stop evolving: During the Pokemon's evolution keep pressing the B button to stop the evolution, also have the Pokemon hold the item everstone no matter what level the Pokemon is it will never evolve if it's holding that item and if it evolves during trade if it's holding the everstone it still will not evolve.
To Stop Evolution:Tap the B Button Repeatedly until Evolution stops (While Evolving)Make your Pokemon hold an Everstone (Does not Evolve while Hold)Note: Trading Pokemon that Evolve can not be stopped.
Wartortle
The evolution of onix is steelix which you can get by trading it with a steel coat
boldore, karrablast+shelmet,