Giant Green Shell
From the Super Mario Wiki
Giant Green Shells are items found in some Mario games. In Super Mario Bros. 3, the green-shelled Big Koopa Troopas wear these giant shells; Mario or Luigi can jump on these oversized Koopas, making them retreat in their shell. As with other turtle 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, if you give a Koopa Troopa a Mushroom, it will turn into a Big Koopa Troopa, and when stomped on, will hide in it's shell. This game doesn't allow you to pick them up.
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.