New Mythical Pokémon have been introduced in Generation VII of the series with Let's Go Pikachu! and Allons Évoli! It was in the form of Steel-type Meltan, which surprisingly had the ability to evolve into Melmetal.
Mythical Pokémon are creatures of Pokémon lore that are said to have been lost or so rare that they have only been seen by a small number of people, hence the name “mythical” beings. They join the likes of Mew, Cerebii, and Jiriachi as ultra-rare to obtain Pokémon.
They became available to catch for the first time in Pokémon GO, disguised as Ditto during an event, but remain extremely rare. However, if you managed to find one, you can transfer the Pokémon to both Let's Go games. And now with Pokémon Home, you can now add them to your team on Pokémon Sword and Shield using Pokémon Home.
However, there is a catch, and it has to do with the evolutionary process. For you to be able to evolve Meltan into Melmetal, the only known way is to give him 400 Pokémon GO Meltan Candy, after which he can evolve. It is currently not possible to evolve Meltan in Let's Go games or Sword and Shield. You cannot return from Sword and Shield either after the first transfer to the game.
This means that if you want to add both Pokémon to your collection, you will not only need to catch two Meltan in Pokémon GO, but you will need to evolve one before sending both to Sword and Shield via Let's Go and Pokémon Home.
It's a long process, and once it will take a lot of time and effort to get it, but Mythical Pokémon don't become available too often and if you want to make sure your Pokédex is ready for the new expansions in the game, you will have to work to get Meltan and Melmetal on your Switch save.
