Sources : Ichneumon

Pliny the Elder [1st century CE] (Natural History, Book 8, 35-37): [Book 8, 35] ...and in the next place she [Nature] has given it war to the death with the ichneumon. [Book 8, 36] That animal, which is also a native of Egypt, is specially known because of this exploit. The asp repeatedly plunges into mud and dries itself in the sun, and then when it has equipped itself with a cuirass of several coatings by the same method, it proceeds to the encounter. In this it raises its tail and renders the blows it receives ineffectual by turning away from them, till after watching for its opportunity, with head held sideways it attacks its adversary's throat. And not content with this victim it vanquishes another animal no less ferocious, the crocodile. [Book 8, 37] ...and the ichneumon watches for it [the crocodile] to be overcome by sleep in the middle of this gratification and darts like a javelin through the throat so opened and gnaws out the belly. - [Rackham translation]

Isidore of Seville [7th century CE] (Etymologies, Book 12, 2:37): The ichneumon is so named from Greek because it recognizes both healthful foods and poisons by its sense of smell. Concerning this animal Dracontius says: The suillus destroys the force of any poison. Moreover, it is called suillus from its bristles (saeta). This beast hunts snakes. When it fights against an asp, it raises its tail and the asp begins to see the tail as the most threatening.When the asp, thus deceived, turns its force on the tail, it is seized. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré, Liber de natura rerum, Quadrupeds 79 [circa 1200-1272 CE]: The beast is called neomon in the Greek language, as Isidore says, because it recognizes both the health and the venom in its food by smell. Of which Dracontius said: the suillus predicts the strength of whatever venom it encounters. It is called suillus because of its bristles, because it has bristles on its body instead of hair. This beast attacks serpents. When this beast fights against asnake, it raises its tail, which the snake sees as a threat. When the snake shifts its strength to the tail, having been deceived it is snatched up by the beast. - [Badke translation/paraphrase]