B Dasher is the name of one of Mario's karts in Mario Kart DS. It's the heaviest of his 3 karts in the game, but it has less Speed and Acceleration than his other 2 karts. Mario is in this kart on the box cover of MK DS. It has the look of a drag racing car.
The body of the B Dasher returns as an unlockable kart part in Mario Kart 7. Aside from having a slightly higher Speed rating, its stats have remained relatively the same.