Here on gyhe desert, we have a lot of shadow people and skinwalkers. There are many Navajo legends of these creatures. Most Natives are very frightened of witches, and skinwalkers are usually known as witches. Each tribe or nation had their own version however and the flip side would be "The Medicine Workers".
Shadow People have been around since time recorded. You can sort of see through them, I have seen them....the disappear fairly quickly when you look directly at them. One thought is that they are an inter dimensional being. Those that see them, might have an ability to see through the viel so to speak...or may be able to see when there is a break in the veil.