Watcher in the Water

The Watcher is a unique being, nowhere else alluded to, and in fact we cannot even say for certain that it was a single creature. It's generally depicted as a many-tentacled squid-like monster, but in fact Tolkien never states this certainly. Indeed, when Frodo asks Gandalf 'What was the thing, or were there many of them?', Gandalf's only answer is 'I do not know.' (The Fellowship of the Ring II 4 A Journey in the Dark.