The Invisible Duck is a ghost-like creature that first appeared the episode "Duck!" in which it was generated when Grim had a good dream. This duck is thus implied to have been created from the subconscious mind of Grim himself.
Invisible to all but the particular person it was haunting at the moment, it would cause that person to be blamed for farting by blowing raspberries while standing next to him.
The Invisible Duck takes the form of a duck with a bluish bill and feet, dark circles around its eyes, a dim bluish glow, and, for some reason, a Pickelhaube (a spiked helmet formerly worn by Germany's military forces) with a large, pale pattée cross on the front).
The Invisible Duck is first seen by Grim, who is sent to jail for exposing himself to children in an attempt to prove he couldn't have farted. Irwin is its next victim, being sent to jail with his dad and grand-mama for disrupting a baseball game. Soon, it causes nearly everyone in town (including even Hector Con Carne (who is "not even on this stupid show any more")) to be arrested. Finally, it tries to haunt Mandy, but she utterly ignores it and all the trouble it causes for her. Though it causes her to be sent to Principal Goodvibes (and against the principal's express orders blows a massive raspberry through the school's PA system), Mandy explains to it that nothing it could ever do could be more torturing or humiliating than having to live life with a "friend" like Billy, horrifying it so much that she causes it to vanish, apparently forever, with an actual, tremendous fart.
The Invisible Duck also appeared in Billy and Mandy's Big Boogey Adventure.
The Invisible Duck was formerly featured as a non-interactive character in CN's online MMO, Cartoon Network Universe: FusionFall, in which it would stalk the streets of Endsville uttering raspberries.