A brief chapter of Alexander Neckam’s De naturis rerum (II, 24) mentions the existence of a fish that lives in the Northern Seas, has only one eye, in the shape of a triangular shield, and is the prey of the white polar bear. This is the very first mention of either of these animals in an encyclopedic text. If the mention of the second is unproblematic, the first is not. We review the different Arctic marine mammals that could possibly hide behind this mysterious description. We identify Neckam’s sources (the Liber monstrorum and the Aeneid) in this passage. We then discuss the monstrous quality of the Cyclops fish. Finally, we study the reception and influence of this chapter of the De naturis rerum. Since this passage was used neither by Thomas of Cantimpré nor by Batholomaeus Anglicus, the Cyclops fish disappeared from encyclopedic texts. That said, this chapter is the probable reason for the inclusion of the white bear in English bestiaries that, until then, were unaware of its existence.
Middle Ages, northern seas, fish, cyclops, polar bear, Alexander Neckam, De naturis rerum, Virgil.