This is Victini, a very special Pokemon introduced for Pokemon Black and White. So special, in fact, that it occupies number 000 in the Pokedex. It's a "phantom Legendary" Pokemon.
Isn't it adorable? Doesn't it make you want to go to Japan in September, buy a copy of the game, and download it from a DS station or game retailer? Coincidentally, that's precisely how you acquire a Victini of your very own.
You'll probably be able to get one in the North American game after its spring 2011 launch, probably through a Toys R Us event or something like that, but do you really want to take that chance? Victini's just a plane ticket (and a couple of months) away.