I thought it was time for different Boo by now. Kaboom delivers! It’s the Boo from Super Mario64 (DS) whom you could find in the courtyard. When you hit it, it would go into a small cage. This was actually a stage called the Haunted Mansion.
A Boo is a common enemy in the Mario series that first appeared in Super Mario Bros. 3. They are a type of white spherical ghost with menacing and constantly open mouths and also small “hands” of some sort. Boos may be a type of ghosts who were once living. While they have been enemies for most of their history, recently they became playable characters and some even allied to Mario.
By Kaboom, info from Mario Wiki