The Ghosts are a group of traditional bedsheet style ghosts that have been appearing in Muppet projects since The Muppet Show, where they debuted in Season 1's Vincent Price episode.
One ghost named Bernard (performed by Peter Linz) appeared in an Oh My Disney video where Rowlf the Dog did a dramatic reading of Grim Grinning Ghosts, with the two deciding to go get coffee at the end.
A large group of Ghosts fill out the ensemble of Muppet Happy Haunts in Muppets Haunted Mansion. Notably, one ghost (again performed by Linz) breaks the fourth wall in the Portrait Hall, reassuring the audience that they won't be explaining all of the jokes.