The cats of Queen Berúthiel were mentioned by Tolkien in a 1956 letter as only one of the two references (besides the names of the Blue Wizards) in the whole Lord of the Rings that did not actually exist, on its own plane (of secondary or sub-creational reality.)[2] Before the publication of the Unfinished Tales in 1980, the cats were (in the words of Christopher Tolkien) "hitherto wholly mysterious."[3]
According to Unfinished Tales, Queen Berúthiel had ten cats. The cats were her slaves whom she used as spies.
“ She had nine black cats and one white, her slaves, with whom she conversed, or read their memories, setting them to discover all the dark secrets of Gondor, so that she knew those things 'that men wish most to keep hidden', setting the white cat to spy upon the black, and tormenting them. No man in Gondor dared touch them; all were afraid of them, and cursed when they saw them pass.[2] ”
Queen Berúthiel was feared and reviled in Gondor, and at last her husband the King banished her from the realm. She was last seen aboard a ship, with all her cats, sailing away into the southern seas.[4]
Many things have been written about the cats of Queen Berúthiel. Tom Shippey says in J.R.R. Tolkien: Author of the Century:
“ Tolkien spent years creating his world, and it shows, in what he reveals and also what he merely hints at. For instance, there are the famous ‘cats of Queen Beruthiel’, the subject of an off-hand comment by Aragorn—we never find out anything more about the cats or their mistress, but just the mention of them suggests that there is a world outside the story.[5]