Middle-earth is peppered with deep horrors, mysterious creatures, and great beasts that lurk and thrive in its diverse biomes. Although many companion pieces for characters and beings in The Lord of the Rings and The Hobbit have been written, there have been some big lore gaps for some terrifying creatures. Of course, the Balrog that fights Gandalf in The Fellowship of the Ring is one such creature with a lot of contextual gaps. There is another creature from earlier in the story that is shrouded in even more mystery, the tentacled terror known as The Watcher In The Water. Not much was known about this creature and its backstory, but books like The Complete Tolkien Companion have shed some speculative light on its potential origins.
Source：https://www.cbr.com/lord-of-the-rings-watcher-in-the-water-explained/
