Shadow creatures are a fascinating subject. I've been a panelist at several sci-fi conventions where shadow people/ghosts were the topic. Some spooky stuff there....
Once, when I was around 15, my friend Dewey was staying over. We woke up and were making up the two beds, talking about TRS-80 programming things when we saw something black appear out of nowhere between the beds and fall towards the floor. I could see Dewey's face and eyes tracking the same object. We both looked and there was nothing on the floor. We looked up at each other and said simultaneously, "Did you see that?"
Both of us thought it was a misshapen bat. There was nowhere for it to go since both box springs were on the floor. It appeared, fell, and disappeared again...and we both saw it.
We were both rather freaked out.
My second shadow creature story took place years later when I married and had three kids. This thing we named Un. It lived in my oldest daughter's room in one corner of the ceiling. Every time the dogs went into her room they would sit down and just stare at that corner. Her room was always colder than the rest of the house by at least ten degrees, quite noticible. My daughter started seeing a young boy, around five or six years of age, almost daily. Un followed us to several houses until one time it apparently got lost on a cross-country move.
Can't say they exist, but I can't say they don't (or are maybe poking into our dimension to see what the weird stupid organisms are doing this week).