Bowser's Shell

Bowser's Shell is an item in the Mario Kart series, starting with Mario Kart: Double Dash!! It is a gigantic spiky shell identical to Bowser's which can crash through multiple drivers and obstacles in a row.

Mario Kart: Double Dash!!
Bowser's Shell first appears in Mario Kart: Double Dash!! as a Special item that Bowser and Bowser Jr. can receive. Like most other Special Items, Petey Piranha and King Boo can also receive it. When released, it bounces around the track like a normal Green Shell, toppling any kart that it hits as well as knocking down the rear character, making the racers drop their items and temporarily disabling them from switching. After 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 being destroyed by some other items, and even bounces off another special item should it collide with one.

Mario Kart DS
This item was also going to appear in Mario Kart DS, and an icon for it is still in the game.

Mario Kart Arcade GP 2 / Mario Kart Arcade GP DX
The item reappears as one of Bowser's items in Mario Kart Arcade GP 2 and as an item any character can use in Mario Kart Arcade GP DX.

Mario Kart Tour
This item reappears in Mario Kart Tour as the special skill of Bowser, Bowser Jr., and Dry Bowser. It acts similarly to its original appearance, though it tends to slide along the middle of a racetrack and curve its trajectory towards nearby drivers. When Bowser's Shell+ is used, the shell is bigger, and will be able to hit other racers more easily.

Mario Kart Tour

 * Tips & Tricks: "This big ol' shell on loan from Bowser will crash any karts it hits and keep on going. It can only be thrown forward."
 * Mario Kart Tour Twitter: "Special item introduction: Bowser's Shell In blue, there are special items for certain drivers. This big ol' shell will crash any kart it hits and keep on going. In tightly-packed races, this powerful item can take out multiple racers in a single throw!"