Historically, the most common squirrel in northwestern Europe was the red squirrel Sciurus vulgaris, and this would likely have been the species prevalent in Middle-earth, or a close relative. The larger grey squirrel Sciurus carolinensis is now much more common, but this is a North American species introduced into Europe in the nineteenth century. It would presumably, therefore, have been entirely unknown east of the Great Sea in the Third Age or earlier.

The etymology of the word 'squirrel' from 'shadow-tail' is clear, but the reasoning behind the name is less obvious. Some sources hypothesise this is due to squirrels raising their tails over their backs, apparently to shade themselves or provide protection. Other sources suggest that this is merely a 'folk etymology', invented after the fact, and that the original reasoning behind the name has been lost.