The article concerns the motif of beaver in the medieval literature. The first part of this study is an attempt at genetic criticism of the antique roots of this motif; we demonstrate a plurality of approaches to describing this animal and a profound connection of early Christian writings and ancient pagan texts. Furthermore, it is impossible to trace, with all certainty, the origins of this topos, as we have shown through source criticism of Aesop's Fables and The Histories of Herodotus. Physiologus and Etymologies of Isidore of Seville, taking from a selection of antique texts, the transformed motif of beaver, so as to adapt it to new guidelines. Since Physiologus was written, until the turn of the 12th and 13th centuries, the ancient plurality of approaches to the description of this animal has been limited and adjusted to requirements of Christian writings. In addition, the analysis of the topos of beaver allowed the voicing an opinion on the subject of the still disputed origins of Physiologus. In the high Middle Ages, the discussion concerned the contradictions between marvellous early Christian literary tradition, the observations of Gerald of Wales and the rediscovered plurality of ancient approaches to zoology. The analysis of Itinerarium Cambriae and medieval encyclopedias along with iconographical material sheds a new light on the origins and functioning of the motif of beaver in medieval writing as well as medieval relations between human and nature in general.