There are no physically real spirit creatures. Whatever a person may believe about the nature of a magical creature, whether appearance or behavior, even places it may reside, they are all non-physical entities. As such, there is currently no way to prove they existence; they are spirits. Thus they fall into the realm of faith, and not that of fact.
Perhaps at some point in the future, that which is ectoplasm (early 20th Century American spiritualism), (the vague concept of spiritual) energy (most current belief systems), Orgone (19th Century psychoanalysis -- thank you Mr. Reich), Virtus (roughly 15th Century England), ki/chi, mana, and whatever else someone may call the 'stuff' of spirit, may be definable. I think at that point, we will have more to learn and discover than we currently imagine.