Get a couple of free mythicals if you have other Pokemon games.

In Pokemon Brilliant Diamond and Shining Pearl, you will be able to get a couple of Mythical Pokemon for free, both available from the old couple in Floaroma Town

If you have save data for Pokemon Let’s Go Pikachu or Pokemon Let’s Go Eevee, you will receive a Mew
If you have save data for Pokemon Sword or Pokemon Shield, you will receive a Jirachi.

These are free gifts only a couple hours into the story. We’ll see if they release other Mythical Pokemon through similar means.

Source 1

Source 2