Delusion

A creature that appears between dreams and reality. Though it can be seen by multiple people at once, no traces can be observed after it leaves. Perhaps it is a product of extreme quantum fluctuations, or some strange "cloud" of information.

All recorded observers have seen the same thing: a white-ish, dog-like shape with eyes like a nebula from the depths of the universe and a shifting number of unnatural shadows in place of a tail.

It is said that if you look at it for too long you will start losing your ability to determine what is reality and what is not. However, nobody has actually seen it for long uninterrupted periods, so this cannot be proven yet.