Bowser's Shell

Bowser's Shell is a special item exclusive to Mario Kart: Double Dash!! that only Bowser and Bowser Jr. can receive (with the exception of King Boo and Petey Piranha that they can also receive). It is a gigantic shell that, when released, bounces around the track like a normal Green Shell, tumbling any kart that it hits as well as dropping the items they hold. After a Bowser's Shell hits the walls of a course roughly fifteen times, the shell breaks apart. Unlike many other items, the bowser shell can resist to be destroyed by some other items if hits them and even bounce off if it collides with another special item.

It is unknown if these are just weapons used by Bowser modeled after him, or actually his spare shells. The item reappears as one of Bowser's items in Mario Kart Arcade GP 2.

Trivia

 * Bowser shells were going to be in Mario Kart DS but were taken out for unknown reasons