Special Item (Mario Kart series)
From the Super Mario Wiki
A Special Item is the name applied to certain item that is obtainable only to a specific character in Mario Kart: Double Dash!!. Unlike a standard item, most of these items can be obtained in any place during a race, and while a character holds an special item, the second character cannot have another special item from Item Boxes.
Mario Kart: Double Dash!!
Special items make their debut in Mario Kart: Double Dash!!. Characters have special items exclusive to them. A certain special item is given to one of the two characters. Though some characters seem to have a personal power-up used only by them, some share the same function like the other's weapon. Petey Piranha and King Boo are the only characters who can receive any special item excluding Green Fireball and Birdo's Egg (due to the Mario Bros. and Yoshi and Birdo each having their own exclusive version of their special item while all other pairs have no difference in items). Additionally in Battle mode, all characters are able to use all the special items, but only Luigi can get a Green Fireball while only Birdo can get a Magenta Egg.
Mario Kart Arcade GP
Special items appear in Mario Kart Arcade GP. They are specific to certain characters with each character having six special items. All special items must be unlocked by winning races.
Mario Kart Arcade GP 2
Mario Kart Arcade GP DX
Special items reappear in Mario Kart Arcade GP DX. However, unlike previous entries, after a character is selected, it is randomly assigned to a special item during a race. Palette swaps of characters will enable the use of an exclusive item only available to that swap.
Mario Kart Tour
Special Items reappear in Mario Kart Tour, working similar to Mario Kart: Double Dash!!