https://www.polygon.com/2018/10/1/17924070/zeraora-pokemon-event-how-to-get

GameStop will give out a special Pokémon starting later this month, The Pokémon Company announced. It’s the mythical monster Zeraora, an electric type that was first shown off earlier this year.

As is often the case with legendary Pokémon, Zeraora’s reveal greatly predated its in-game debut. The Pokémon Company introduced it with a short teaser in April. Ahead of the release of a new trading card game expansion featuring the Pokémon, however, players can get Zeraora for their games via a download code.