Mice

Small, long-tailed rodents commonly considered to be pests. No mice are ever seen directly in the histories of Middle-earth (though they were reported to have been found in Bag End after the defeat of Sharkey). Nonetheless, they are referred to metaphorically several times, and seem to have been well known and presumably widespread in the Third Age. They were certainly known in the Shire, because Sam chose the mouse's grey colour as part of his rhyme about the Oliphaunt. Gollum knew about them, too, and compared his capture by Frodo and Sam to a mouse being pounced upon by cats (though Gollum characteristically called these creatures 'mices'¹).