Giant Green Shell
From the Super Mario Wiki
Giant Green Shells are items found in some Mario games. In Super Mario Bros. 3, green-shelled Big Koopa Troopas and Colossal Koopa Paratroopas wear these giant shells; Mario or Luigi can jump on these oversized Koopas, making them retreat in their shell. As with other Koopa shells, they can also be picked up and kicked. They return in New Super Mario Bros. U and New Super Luigi U, where they serve the same purpose. In Super Mario Maker and Super Mario Maker for Nintendo 3DS, if a Koopa Troopa is given a Super Mushroom, it will turn into a Big Koopa Troopa, and when stomped on, will hide in its shell. They cannot be picked up in this game, but can be used to break through Brick Blocks and Hard Blocks without rebounding.
Giant Green Shells make an appearance in the Mario Strikers series as an item, along with their smaller counterparts. Working similarly to Bowser's Shell from Mario Kart: Double Dash!!, they can be collected and fired. The item spins into opposing team members, squishing and stunning them for a short while. After hitting a player, it still bounces around the stage until it wears out.