River-tolls

"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."

—The Hobbit, "A Warm Welcome"

River-tolls were a type of toll collected by either the Wood-elves or the Lake-men. These river-tolls were occasionally a source of contention between these two group as they had squabbles over whether they could collect or had to pay these river-tolls for passing down the Forest River.

In adaptations

As a result of the similarities between the words river-toll and river-trolls, some readers mistakenly read the word "river-tolls" as "river-trolls", thereby imagining an entirely new creature which Tolkien never intended to exist. As such, these types of trolls never actually appear in J.R.R Tolkien's writings. As a consequence, many adaptations have included river-trolls as being actual creatures that existed within Middle-earth.

In the video game LEGO The Hobbit: The Video Game, River-trolls are actually added as part of a DLC as playable characters, thus making it possible that the game designers were either among those who fell for this common misreading or poking fun at this misreading.

River-trolls can be found in some fantasy games, Beasts of War and RuneScape, for instance.