|“||Thorin and Fili and Kili and the hobbit went along the shore to the great bridge. There were guards at the head of it, but they were not keeping very careful watch, for it was so long since there had been any real need. Except for occasional squabbles about river-tolls they were friends with the Wood-elves.||”|
River Trolls are never mentioned in J.R.R Tolkien's writings. The idea that Tolkien wrote about them arose from a misreading of the text in The Hobbit. As the dwarves enter Lake-town, the narrative mentions "occasional squabbles about river-tolls" between the men of Lake-town and the nearby elves. 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 entirely new creatures called "River-trolls" that Tolkien never intended.
River Trolls can be found in some fantasy games, Beasts of War and RuneScape, for instance.