Onix has been one of the most popular Rock-type Pokémon since the beginning of the series because Gym Leader Brock has one. That first gym battle ending in a battle against the rock snake felt a lot cooler on Game Boy Color back then than it looks now, probably, but the anime probably had something to do with that. Regardless, in generation two, Onix got evolution to a Steel-type called Steelix. Here is how to evolve your Onix into a Steelix in Pokémon Go.
To evolve an Onix into Steelix in Pokémon Go, you will need to gather a couple things. First, like all other evolutions in the game, you will need candies. Onix ones in this case. Keep catching and transferring Onixs until you have amassed 50 candy.
Next, you need a Metal Coat like in the games. Metal Coats are most easily obtained by spinning a Pokéstop seven days in a row. On that seventh day, you are guaranteed an evolutionary item. Additionally, you can potentially get a Metal Coat from getting your Field Research done after seven days, and Pokéstops have a very slight chance to give you one in normal circumstances.
Unfortunately, there is no guaranteed way to get a Metal Coat, but once you get lucky, you are ready to go. Give it to Onix along with the candies, and you have a shiny new Steelix ready for battle.