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). It is a gigantic shell that, when released, will bounce around the track like a normal Green Shell, tumbling any kart that it hits (unless the kart is star-powered). After a Bowser's Shell hits the walls of a course roughly fifteen times, the shell will break apart.

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.