With the Yoshi Chase promotion various QR codes will be released where Nintendo 3DS owners can bring their handheld to select Asda stores following the games release, scan the code and have a new playable Yoshi in their game. There are 30 Asda stores participating in this promotion so be sure to check if yours is, pending you want to partake that is.
The Black Yoshi is the first new playable Yoshi being released the same day as the game, May 25th. White, blue, and red Yoshi variants are in the works and will be released at a later date. In-store sampling teams will be handling the QR codes between the hours of 9:30am and 5:30pm, Friday the 25th and Saturday the 26th, and can only be activated there.
This promotion has only been confirmed for Europe, but with any hope Nintendo will replicate a similar promotion in North America (I’d assume at Wal-Mart).