"The creature was almost the size of a rabbit, but much thinner"

European pine marten
GNU Free Documentation LicenseEuropean pine marten - Credit: Dani Kropivnik
Gwin is a European pine marten (Martes Martes). If you're in possession of a Chicken House edition of Inkheart with the Inkfolio at the back, you can turn to page 552 to get some more information about these cute creatures. If not, or if you don't trust yourself to be turning to the end just yet, everything you could ever need to know about Pine Martens can be found right here.