It melts particles of iron and other metals found in the subsoil, so it can absorb them into its body of molten steel.
– Pokémon Let’s Go Eevee Pokédex
Meltan was introduced in Let’s Go Eevee / Let’s Go Pikachu, even though they can only be caught in Pokemon Go. Because of this, I went back to playing Pokemon Go for a bit, so I can catch enough Meltans to get a Melmetal.
Meltan is introduced to have some sort of connection to Ditto. Indeed its body looks like a metallic version of Ditto. It’s very small, only 0.2m (0’08”) tall, tied for smallest Steel type Pokemon.
Stat-wise, it’s pretty weak for a mythical Pokemon, with only 300 BST. Its Attack and Defense are its highest stats.