From the Super Mario Wiki
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Big Steelies (also referred to as "bowling balls" in Mario Mania Player's Guide) 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 are mentioned in Super Smash Bros. Melee in the Koopa Clown Car trophy, once again labelled as "bowling balls".