If you'd like this glorious prancing beast to smite all your rival trainers with Hyper Beam, then you will be pleased to know that Nintendo are giving Arceus away for free, again.
Arceus is literally a god when it comes to other Pokemon. It was the creature, in the wonderful world that is Pokemon, that hatched out of the first egg and then created the three Pokemon which had the ability to control time, space and antimatter.
Arceus is a big fucking deal, basically. So the fact that Nintendo is giving Pokemon God away until the end of the month is probably worth the attention of any Pokemon fan, especially if you missed it when Arceus was being given away earlier this year. The new mythical Pokemon this month is actually Meloetta, a normal/psychic type that becomes a normal/fighting beast once you use Relic Song (which you get from the Move Tutor in Mauville City).
There is the small quirk where it's for Pokemon X/Y/Alpha Ruby/Omega Sapphire and not Sun or Moon, but being able to pulverise any Pokemon into literal space dust with the power of a thought (or whatever else it is that lets you bend time, space and anything else) seems like good fun.
If you'd like to get Arceus into your team, you'll need to use ARCEUS2016 codes through the Nintendo Network. Simply navigate to Mystery Gift from the Start menu, then hit Receive Gift. From there, hit Get With Code and pop in either of the codes above. (If ARCEUS2016 doesn't work, you might want to give ARCEUS20 a shot. That's working for North Americans, at least.) You don't need to input a code for Meloletta: just go Mystery Gift, Receive Gift, then Get Via Internet.
Either mythical Poekmon will then be available from the delivery girl in any Pokemon center, and you're good to go. Remember: it's only for the older Pokemon games and Meloetta is only available until Christmas Eve, while Arceus will hang around until December 31. But hey, there's worse ways to spend Christmas than smashing trainers with Pokemon God and a singing bard.
Update: Fixed up the process for getting either Pokemon into your team. Sorry for the confusion!