The Medical Shadow, one example of the 5 Nightmare Shadows
Five troublesome dream spirits, they gather fear energy from children's nightmares. These specters enter a child's subconscious, dig out their greatest fears, and turn them into surreal nightmares. Being non-corporeal, they cannot be destroyed, but can be dispelled by a particularly brave child. There favorite target is poor Ned Needlemeyer. However, they don't always attack only Ned, as they have even managed to give terrible nightmares to Max with his Were-Goofy dream, to Simba with his recurring nightmare of Mufasa's death and its latest variation with the shapeshifting Scar apparation, and even Pluto when he encountered the Judgement Day Cats.
However, when Ned dispelled them, they were revealed as some people Ned was familiar with. Ned had managed to push them out and replace them with benign representations of his fears, allowing him to confront the true enemy.