Sources : Ferret

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Quadrupeds 4.42): A furunculus is an animal that we commonly call a 'fury'. It has a color between white and brown, is a little larger than a weasel, and is very similar to a sable. It is therefore a spirited animal and wild beyond what nature permits it to do. For this purpose furunculus were taught to be placed by hunters in the caves in which rabbits hide. They boldly penetrate the pits and attack the beast with their bites, until they throw it into distress and expel it into the net. Nor do they hate this beast only, but they attack whatever animal seems to breathe, no matter how great it is, because of their strength. Nor is this because of a desire for meat, on which they certainly do not feed, but with a desire to injure, or, as some say, because of the blood of animals, which they certainly willingly receive in drink. A furunculus is fertile at birth, birthing seven or eight children at once. If a female has no mate, when lust is stimulated, she swells up and dies. She carries in the womb forty days. - [Badke translation/paraphrase]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book18.74): A Firet is called Migale [a name also used for the shrew], & is a little beast, as it were a wesell, and is a glutton & guilefull, and a ravener: for wt guile he ravisheth what he will eate afterward, as the Glose saith, sup. Leu. 11. For dred he saineth himselfe mild, when a man commeth nigh him, but he biteth anone. & sheddeth venimme, as it is sayd ther Arist[otle] speketh of this beast Migale & faith, that he hateth horses & mules, & grieveth them, & lieth specially in await on a mare that is with foale, & fighteth against Serpents, and armeth him with rewe, as he saith. - [Batman]