Boo

Learn about Boo behavior in Super Mario World: they freeze when you look, chase when you turn, and can't be defeated. Some turn into blocks.

Boos cannot be defeated. To get past them, keep an eye on them and run when you have an opening. You can also cape spin (use the cape feather to spin) onto them and bounce off unharmed.

In one ghost house (a type of level), some Boos transform into blocks when you face them, which can then be used to jump on.