A Metal Coat is a held item in Pokmon GO that can be used to evolve certain Pokmon, such as Scyther into Scizor and Onix into Steelix. It is a relatively rare item, but there are a few ways to obtain it.
One way to get a Metal Coat is to spin PokStops. Metal Coats can be randomly awarded as a reward for spinning PokStops, but the chances of getting one are very low. You can also get a Metal Coat by completing Field Research tasks. Field Research tasks are special tasks that can be obtained from PokStops, and some of them reward players with a Metal Coat upon completion.