Cactus

The Cactus is an unused object, likely meant to have appeared in Shifting Sand Land. It is a cylindrical cactus with spikes and does damage when touched. It is currently unknown why this went unused, possible due to being used in an unused desert stage like Dry Bones Desert, and was not placed in Shifting Sand Land.

Irrational Fear
The players who have found the cactus in Super Mario 64 have sometimes claimed to develop a strange fear of spiked objects, and have had nightmares of the cactus rolling towards them. It is currently unclear why this happens, though it is possible it is a side-effect of encountering the cactus, similarly to the Wario Apparition's side-effects when talked about, or the Kellogs Penguin Shared Nightmares.