The Pokémon Company has now revealed that Meltan evolves into Melmetal, making it the first Mythical Pokémon to be able to evolve.
Here's the official bio for the 'mon:
Melmetal was worshiped in ancient times as a Pokémon with the power to create metal. Because of its Evolution from Meltan, its body has hardened and is now incredibly sturdy. Melmetal is made of liquid metal, and its flexible arms can be whipped forward to deliver a devastating punch. It can also execute a powerful, unique Steel-type physical move known as Double Iron Bash. By using the hex nut in its chest like an axle to spin its arms around, Melmetal can deal a double strike to its opponent. The weight of the heavy hex nuts on its arms combined with the spinning motion gives the Double Iron Bash move extraordinary power.
To evolve Meltan into Melmetal, you'll need a whopping 400 Meltan Candies. You'll also be able to gain Meltan Candies by sending that Pokémon from Pokémon GO to the upcoming Switch title Pokémon: Let’s Go, Pikachu! or Pokémon: Let’s Go, Eevee!, when it eventually launches.