Giant Green Shell

From the Super Mario Wiki

Giant Green Shells[citation needed] are items found in some Mario games. In Super Mario Bros. 3, Green Gargantua 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, though they can also break through normal-sized Brick Blocks, rather than bouncing off of them.

In Super Mario Maker, Super Mario Maker for Nintendo 3DS, and Super Mario Maker 2, if a green Koopa Troopa is given a Super Mushroom, it will turn into a Big Green Koopa Troopa, and when stomped on, will hide in its shell. While they work as they do in the New Super Mario Bros. games, they cannot be picked up in these games, but they can also break through 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.