Bowser's Shell

From the Super Mario Wiki

A Bowser's Shell is a special item exclusive to Mario Kart: Double Dash!! that only Bowser and Bowser Jr. can receive. When released, it will bounce around the track like a normal Green Shell, tumbling any kart that it hits (unless the kart is star-powered or invisible). 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.

Shells
Green Shell | Giant Green Shell | Red Shell | Giant Red Shell | Triple Shells (Green · Red) | Blue Shell | Giant Blue Shell | Yellow Shell | Spiny Shell (Mario Kart) | Giant Spiny Shell | Black Shell | Bowser's Shell | Gold Shell

Character's Items from Mario Kart Arcade GP 2 Mario - Luigi Fireball - Hammer - Chain Chomp Donkey Kong Barrel - 10T Hammer - Giant Banana Bowser Bowser's Shell - Flaming Tending - Thwomp Toad Poison Mushroom - Mushroom Hammer - Mushroom Powder Wario Snot Bomb - Stretch Punch - Bob-omb Yoshi Rolling Egg - Wacking Pan - Tongue Princess Peach Peach's Bird - Peach's Frying Pan - Piranha Plant Pac-Man - Ms. Pac-Man Pooka - Rally X - Galaga Boss Blinky Petit Monster - Monster's Hammer - Flare Ball Waluigi  ??? - ??? - ??? Mametchi Bagubagutchi - Mame Block/Present - Mame Drink

Mario Kart Items

Absorbing Mushroom | Banana | Basin | Birdo Egg | Black Shell | Blooper | Blue Shell | Bob-omb | Boo | Bowser's Shell | Cape Feather | Chain Chomp | Cream Pie | Driver Virus | Fake Item Box | Fireball | Giant Banana | Golden Mushroom | Green Fireball | Green Shell | Mega Mushroom | Mini Mushroom | POW Block | Peach's Frying Pan | Red Shell | Shield | Smokescreen | Star | Super Mushroom | Tacks | Thunder Cloud | Thunder Stick | Thunderbolt | Time Bomb | Triple Green Shells | Triple Red Shells