Recently The Pokémon Company International had announced Meltan, a new Mythical Pokémon which first appeared in Pokémon Go and can get sent to the upcoming Let's Go games by using the GO Park Complex in the games Let's Go Pikachu and Let's Go Eevee.
Using a comparison to the games Colosseum/XD, where you can catch and battle Pokémon, Munchlax and Bonsly both appeared in the Generation 3 game Pokémon XD, yet they were Generation 4 Pokémon; this means that it's possible that this Pokémon is a possible candidate for the 8th generation.
A while after the announcement of Meltan, the Pokémon Company International made a video together with Niantic in which some mysteries regarding the Pokémon got cleared up, but there were more mysteries regarding this Pokémon. As Game Freak had said a new Pokémon would be introduced. Due to Meltan's dex number being unknown, we won't know what generation Pokémon this will be. [Note: Presently, Pokémon GO datamining here showed the the Dex ID for both Meltan and Melmetal were 808 and 809 respectively, but it could just be temporary internal numbering]
After the announcements of Meltan, The Pokémon Company International seemed rather quiet, until another video popped up in which they announced yet another Pokémon.This time it was the evolution of Meltan, namely Melmetal. This is an all new situation because no Mythical Pokémon to date has evolved. The only other exception to the Mythical formula, was regarding Manaphy and Phione: Manaphy could breed Phione, but Phione could not evolve into Manaphy.
Meltan is only able to evolve into Melmetal as of now, by using 400 Meltan candies on Meltan in Pokémon Go. It's unknown as of yet if this will change in the future. If it doesn't, it means it would be really hard to get a Melmetal; which is usual with mythical Pokémon. According to The Pokémon Company it may not change which could indicate that in future games it will be especially hard to get this Pokémon.