It is the last Pokémon in the National Pokédex as of Let's Go, Pikachu! and Let's Go, Eevee!.
It was officially revealed on October 24, 2018.
Melmetal is a large, silvery Pokémon with a humanoid build that consists mostly of liquid metal. Many dark gray hexagonal nuts are embedded in or attached to its body: one around each shoulder, one around the center of each arm, one in each hand, one in each foot, and a small one in the center of its torso. A large, golden hexagonal nut encircles its neck and a smaller one acts as its head. Floating in its head is a small, dark gray sphere. It has a small red tail resembling an electrical wire.
Melmetal is a Pokémon that has come back to life after 3,000 years. In ancient times, it was worshiped for its ability to create metal from anything, which was used to create unique metal tools. According to Professor Willow, Melmetal was considered a sacred being that lived in a secluded area deep within a wooded land that was full sedimentary rocks rich with magnetite. Melmetal can split itself into multiple Meltan. The process of evolution has hardened its body, making it incredibly sturdy. However, it is still in a liquid state and has some flexibility, particularly in its arms. Melmetal is the only known Pokémon capable of learning the move Double Iron Bash.
This makes Melmetal, along with its pre-evolved form Meltan, the only Pokémon in Pokémon: Let's Go, Pikachu! and Let's Go, Eevee! whose Kanto Pokédex number (153) is not the same as its National Pokédex number (809). While Kanto Pokédex numbers are normally used throughout the game, the judge screen displays National Pokédex numbers, which is evident for Meltan and Melmetal.
Melmetal has similar design aesthetics to its pre-evolution, Meltan. Its head and features may be based on hexagonal nuts and its tail resembles an exposed wire. Its body is based on mercury, a metallic element that is liquid at room temperature. Mercury can also conduct electricity, as well as react with many other metals to form amalgams.