To prepare for the release of the upcoming Pokemon Black and White, from the 7th February you'll be able to download not one, not two, but three legendary pokemon via the Nintendo Wi-Fi Connection. And they won't be just any old pokemon - instead, these legendary forms are what people refer to as 'shiny', meaning they're in alternate colours to their usual forms (making them rather rare). The pokemon in question all originate from Pokemon Gold and Silver - Raikou is an electric pokemon said to represent the lightening strike that hit the burning 'Brass Tower', Entei is a fire beast, who represents the flames that burned the tower, and Suicune, a water pokemon that represents the rain that fell, putting out the burning tower. The pokemon also know moves that they wouldn't normally learn - for example, Raikou can use the weather to his advantage with the Weather Ball move, Entei has Flare Blitz to engulf his foe in flames, and Suicune can use it's Sheer Cold move for a one-hit kill.
Once you've recieved the mystery gift, pop to the nearest PokeMart in the game, and have a chat with the man in the green suit and hat, who'll be more than happy to hand over your new pokemon. The pokemon will be released one at a time, and be available for one week only, before the next one is available. Raikou is first, from the 7th February to the 13th, then Entei from the 14th February until the 20th, and finally Suicune from the 21st February through to the 27th. So put the date in your diary, and get ready to expand your collection!