I was once sleep deprivated and seen black dog. Told my friends about this halucination... they asked if I read Harry Potter. I said yes. And laughed.

I see **** at times... and I do joke about it still. Sometimes I think I must be making it up and it's real (in one camping place I seen frogs crawling on walls and mirrors. Sure they were real, unless the whole bus of our tour hallucinated).

Hard to tell. Also... I sometimes wonder if hallucinations aren't glimpses of something that exists, but most people filter out.