It doesn't evole at a level, it evolves into Steelix when traded holding a Metal Coat.
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The onix you found was a very VERY rare one. It is called a shiny Pokemon and the star represents that it is a shiny Pokemon.
You must Trade Onix while it is holding a Metal Coat in order to Evolve into a Steelix.
Pokemon Shiny Gold is a computer Rom hack of Fire Red an emulation remake of GSC most Pokemon evolve different ways Graveler evolves at level 40 and scyther evolves with a moon stone, i don't know all of them but look around for some more evolutions. By Nicholaszero8 I've played the hack. I KNOW.
Onix evolves into Steelix when it is holding a Metal Coat and traded to someone else.
Pokemon Tower Defense is 1st generation pokemon only, so no fourth generation or second generation evolutions (including Steelix). Therefore, Onix can't evolve.