Nintendo and The Pokémon Company have been distributing mythical, legendary, and event-only Pokémon all year long as part of the celebration of the franchise's 20th anniversary. Arceus was distributed earlier in the year, but if you missed out, you've got a second chance. Arceus is once again being distributed in sixth generation games in North America and Europe.
You can nab the Alpha Pokémon by entering the code "Arceus20" in your game in North America or "Arceus2016" in Europe. This will only work if you didn't already download Arceus earlier this year. These codes are only valid until the end of the calendar year, so make sure you don't miss your chance!