From the Super Mario Wiki
A Giant Green Shell
sprite, along with its other color.
Giant Green Shells are items found in many Mario games. In Super Mario Bros. 3, Mario or Luigi can jump on a Gargantua Koopa Troopa, who then hides in its Giant Green Shell, or Giant Red Shell.
Giant Green Shells make a return in Mario Strikers series as an item. Working similarly to the Bowser Shell item from Mario Kart: Double Dash!!, when used, they will shoot out towards the opposing players. Even after it hits a player, the item then continues to bounce around the stage until, at last, it wears out, and falls apart.
Giant Green Shells also appear in New Super Mario Bros U and New Super Luigi U. The player can jump on a Gargantua Koopa Troopa, who then hides in its Giant Green Shell, or Giant Red Shell.