Gandalf mentions them in the books but refuses to give further explanation
“There are many evil and unfriendly things in the world that have little love for those that go on two legs, and yet are not in league with Sauron, but have purposes of their own. Some have been in this world longer than he.” - Fellowship of the Ring
"Far, far below the deepest delving of the Dwarves, the world is gnawed by nameless things. Even Sauron knows them not. They are older than he. Now I have walked there, but I will bring no report to darken the light of day." - Two Towers
What do you think these things are? GeekZone has a couple of theories in his video:
Agree or disagree? Do you have your own theories about them?