There are numerous mentions of rats in Tolkien's tales, but those references are almost always metaphorical in nature. There is in fact only a single direct reference, when Hobbits searching Bag End for Lotho Sackville-Baggins after the defeat of Sharkey could find only mice and rats. Nonetheless, the fact that many different characters refer indirectly to rats - if only as an insult or a simile - shows that they were commonplace across Middle-earth.

Words similar to 'rat' occur in many different languages across at least the Celtic, Germanic and Romance families, implying that the word comes from a very ancient root indeed. The details of that root remain unclear, but many sources suggest connections to words for 'gnaw' or 'scrape' going back to archaic Proto-Indo-European.