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River Trolls are never actually mentioned as such in J.R.R Tolkien's writings, but some readers think they are because of a misreading of the text in chapter ten of The Hobbit, as the dwarves entered Lake Town, where "occasional squabbles about river-tolls" between the men of Lake Town and the nearby elves are mentioned. In other words, squabbles over whether the elves or the men could collect (or had to pay) tolls for passing down the river. Some readers mistakenly read the word tolls as trolls, thereby imagining an entirely new creature called "River-trolls."
River Trolls can be found in some fantasy games, Beasts of War and RuneScape, for instance.