A shadow creature is similar to a shadow person but not a human form. But what is it? That is a good question. A shadow person in my opinion is a negative human energy. Some have ideas that they are also demons. I have yet to come across a true "demon"... although I have had experiences of an entity to claim to be the Devil. I think there are some bad people out there and this continues in the afterlife if for whatever reason they do not move on. Without a human physical form, energy- good or bad, can take many forms. I believe that negative energies are people that are stuck that want to bully others as they did when they were living.
I have also seen other types of shadow creatures, that I don't believe are human at all. They actually appear to be animals. I had an experience with a creature, I want to call a shadow dog. Now, when I was a kid 12+ years old, I had a black cocker spaniel. Many times in my home I thought that my dog was darting across the floor but she was not there. This would happen a lot while sitting in the living room at night with all the lights off, while watching TV. This thing was fast moving, no consistency in pattern and sporadic. Come to find out, my sister and father had also seen this in the house. They all thought that it was our dog at first, but she was either not in the room or was lying on the couch with us. This thing was not scary, just surprising.
Another account was when I was living in an apartment. It was actually my daughter that had the experience. My daughter was about 4yrs. old at the time. She told me that there was a dog that guarded the bathroom at night, it was all black. It would walk back and forth in the hallway in front of the door. She was scared, but would wait until the dog was furthest away from the door and quickly run into the bathroom. She still remembers it to this day.
It is possible that these are animals or pets that have passed on. Why they look like shadows, I do not know. Maybe it's possible that it's we that only perceive them as shadows and not what they truly looked like in real life. Could be due to our lack of ability in clear perception to these energies. Or the energies are not strong enough to be clear to us on the physical plane. Whatever the reasons that hold these people and creatures to this plane of existence, it seems they are stuck until they find whatever they are searching for.