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 that ended in a battle against the rock snake looked a lot cooler on Game Boy Color then than it probably looks now, but the anime probably had something to do with that. Regardless, in Generation II Onix got an evolution into a Steel-type called Steelix. Here's how to evolve your Onix into a Steelix in Pokémon Go.
To evolve an Onix in Steelix into Pokémon Go, you will need to collect a couple of things. First, like all other evolutions in the game, you will need candy. Onix in this case. Keep catching and transferring Onix until you have accumulated 50 candies.
Next, you need a Metal Coat like in the games. Metalcoats are most easily obtained by spinning a Pokéstop for seven days in a row. On that seventh day, you are guaranteed an evolutionary item. Additionally, you can potentially earn a Metal Coat by completing your Field Research after seven days, and Pokéstops have a slim chance of giving you one under normal circumstances.
Unfortunately, there's no guaranteed way to get a Metal Coat, but once you're lucky, you're good to go. Give it to Onix along with the candy and you have a shiny new Steelix ready for battle.