Update More details on this new, horse-like Pokemon on August 20
Update (August 20): Nintendo has officially confirmed the Keldeo event, in addition to some exciting new details. Once acquired, Keldeo can then be transferred over to Pokemon Black or White Versions 2. Once transferred, players can partake in a "scenario" that will transform Keldeo into a new "Resolute Form" and teach it its "signature move" Secret Sword.
In order to acquire Keldeo, make sure you visit a participating GameStop location between August 27 and October 6. Then make sure you follow these simple requirements:
A Nintendo 3DS or Nintendo DS system
A copy of the Pokémon Black Version or Pokémon White Version game
The Pokédex within your game
No more than 11 Wonder Cards in your possession
Pokemon Black Version 2 and Pokemon White Version 2 both release on October 7 for the Nintendo DS handheld.
Starting August 27 and running through October 6 GameStop will be running a promotion, giving away a free, original mythical Pokemon via special distribution. Who's that Pokemon? No one yet knows, as all that's been provided is a the new Pokemon's silhouette. If you're excited to see just what sort of Pokemon it is, full details will be shared on August 20 at the Pokemon website.
The new Pokemon will be the ninth special event release for the fifth generation of Pokemon. Black and White have received several epic giveaways, from Darkrai to Zoroark, through the Global Link promotional system. This next event won't provide its Pokemon for free to everywhere, however, you'll be required to go to a local, participating GameStop -- unless there's a change down the line.
Pokemon White and Black Version 2, coincidentally, will be released on Oct. 7, the same day this mythical Pokemon event ends. As you'll have to head into GameStopanyways, you'll be in perfect position to preorder these exciting games. Nintendo has it all figured out. Remember, once a GameStop employee smells preorder on you, they won't back down. It's best you leave the store quickly before someone gets hurt.
Edit: As pointed out very quickly in the comments, it appears the Pokemon is Keldeo, originally outed as early as February this year. Still, Keldeo is an event exclusive Pokemon, and -- correct me if I'm wrong -- has yet to have an official release event yet. Thanks!