Invisible Duck
Ghost Duck
Classification Duck
Real world information
Appears in The Grim Adventures of Billy & Mandy
First appearance Duck!
Voice actor Wikipedia:

The Invisible Duck ghost first appeared the episode "Duck!". It kept making people think that the person that sees it first farted. It does that by doing raspberries. It was first seen by Grim, who got sent to jail for people thinking it was him. Then next was Irwin who was also sent to jail with his dad and grandmama. Then it was Harold who saw it. It tried to haunt Mandy last, but she was unimpressed and unaffected throughout it's entire treatment of her. After a talk with the principal and making a very long raspberry noise through the School's PA system, Mandy explained to it firsthand that nothing it can ever to do her will be more torturous or humiliating than having to live life with Billy as her "friend", going into so much detail as to what kind of horrors she experiences on a daily basis that she scared it off from haunting people seemingly forever. Before the Duck disappeared, it made a real fart out of nervousness from how intimidating Mandy was and how scary she described Billy's behavior.

The Invisible Duck also appeared in Billy and Mandy's Big Boogey Adventure.