From the Super Mario Wiki
A Mystery Crate is a object found in Yoshi's Story. It is one of the most common objects in Yoshi's Story. To open the Mystery Crate, a Yoshi has to Ground Pound on the top of the Crate. These crates usually contain Melons. These items can be also pushed. There is also an enemy called an Air Bag that is disguised as a ordinary Mystery Crate. However, the disguised enemies can’t be pushed.