hallucination or Wraith Shadow creature?

What you seen is called a shadow person. Although I've never heard of someone seeing them directly only out of the corner of their eye. You might want to check your family history. I've seen and had weird things happen to me my whole life only as an adult did I find out that my grandmother seen a lot of odd things too and, knew things she couldn't have. In short I know what they are because I've seen them. Everything I seen I can verify its real because others have recorded their experiences too. Have a blessed day.