Why would they necessarily be demonic ?
We have life underground. We have life in the deepest parts of the ocean.
We have life in the air. Why would we not have life that can move between the upper atmosphere and the edge of 'space' ?
We have creatures on land and ocean that can make themselves almost invisible. Why couldn't there be creatures high above who can do the same ?
I believe that biologist and author Ivan Sanderson speculated that some of these ‘things seen in the skies’ might actually be forms of life indigenous to Earth which we simply don’t recognize for what they are.
I’ve always thought that was an interesting idea.