YouShouldBeAfraid/Shared Nightmares

Shared Nightmares are anomalies where multiple people have similar nightmares about Super Mario 64. It is currently unclear why these happen, though it is possible the causes have a strange aura, similarly to Wet-Dry World.

Beta Penguins
When an image of early versions of the penguins of Super Mario 64 are viewed, several people have reported having nightmares of being chased by these penguins, and them making loud screeching sounds. If seen, it is advised to look to your feet and not acknowledge them. The most common of these beta penguins to give nightmares are penguins seen on an old Kellogs promotion with Nintendo that had strange, unnerving models of the penguins on boxes.

You should be afraid.

Mad Piano
People who have encountered the Mad Piano have reported nightmares that have had the Mad Piano in them. It is currently unclear if this is paranormal, or instead due to many considering the Mad Piano to be scary.

Wario Apparition
People who have encountered the Wario Apparition have claimed to have nightmares about the Wario Apparition. However, due to the nature of the side-effects of describing the Wario Apparition, not many have said much about the nightmares, though it is safe to assume that the dreams would have been the people running from the Wario Apparition in the Dire, Dire Docks hallway.

Cactus
People who have encountered the Cactus have claimed to have nightmares of the cactus rolling towards them. It is currently unknown why this happens, though it is often tied to the sudden fear of sharp objects that manifest after encountering the cactus.