From the Super Mario Wiki
Big Steelies are large iron balls that appear in Super Mario World in the last battle with Bowser. He drops these from his Koopa Clown Car. Mario can jump over them to avoid getting hit. Mario can also spin jump on them without getting hurt. It can be destroyed if Mario uses his sliding cape attack against it using a Cape Feather but it will show a glitched sprite.
Big Steelies also appear in Super Mario 64 and Super Mario 64 DS, in Bob-omb Battlefield, Tall Tall Mountain, Tiny-Huge Island, and, in the DS remake, Battle Fort. They come out of holes and roll in the ground. If Mario gets hit by one, he will lose two wedges of health. In the original game, they are indestructible, but they can be destroyed with a punch from Wario or a Super Mushroom in the remake.
In the Mario Mania Player's Guide, they are referred to as "bowling balls".