Like the basilisk, with which it was sometimes identified, the fabulous creature known as “cockatrice” was formerly supposed to be able to kill man or animal just by a glance from its deadly eyes.
The actual existence of such a creature was believed even into the seventeenth century. It was then thought to be a kind of serpent; so considered by Shakespeare (in Romeo and Juliet) and by the translators of the King James Version of the Bible (in the Psalms and in Jeremiah).
It could be killed, people said, by the sound of a crowing cock, so travelers were wont to carry a rooster with them when going through regions where they were thought to dwell.
The only mammal unaffected by the baneful eye of the creature and which could attack it successfully was the weasel, because this animal could cure its own injuries by rue, the one plant which the cockatrice could not wither.
Like the basilisk, the cockatrice was believed to hatch from an egg laid by a cock, a belief probably influenced by the first part of the name.
But cockatrice is actually a corruption of the Latin calcatrix, and this, in turn, was a translation of the Greek ichneumon. This was and still is an Egyptian quadruped resembling a weasel.
It is a mortal enemy of the crocodile, devouring its young and searching for its eggs.
Hence its name, for ichneumon means “a tracker.” It was anciently believed that when the ichneumon found a crocodile asleep, it would dart into its open mouth, into its stomach, and kill it by eating through its belly, but this is not true.
So little was actually known of the creature that any fantastic story was accepted by credulous persons.