Plumber Boo is a ghost that resembles everyone's favorite plumber: Mario! Nobody knows where he got the hat, but we know it can't be Mario's. Can it?!?! You can't catch this boo, it's just a distraction. It doesn't cause any damage to you, either. Unless, of course, you go dashing off into unexplored rooms following it, in which other Ghosts can harm you. (Don't DO IT!) You can hear Mario music when one appears. If you follow it for 30 seconds straight, it will start dropping money.
Mario ghost

The Plumber Boo.