The medieval basilisk was vastly different from its descendent in Harry Potter and the Chamber of Secrets. Rather than a monstrous serpent (which was called a wyrm in the Middle Ages), the basilisk was a ludicrous cross between a serpent and a chicken. Closely resembling, and often confused with, a cockatrice, the basilisk had the head of an angry cockerel, the body of a snake, and the feet and wings of a chicken. Despite its poultry nature (sorry, couldn’t resist), the basilisk was seen as the king of serpents, for its name derived from the Greek basiliscus meaning ‘little king’.
The basilisk is born when a serpent’s egg is incubated and hatched by a cockerel. It does not even need to touch someone to kill them: its look, hiss, and scent are sufficient, and as a result people flee upon encountering one. It lurks in the desert, and when it spots a thirsty man heading for a river will give him hydrophobia and send him mad. Despite being so deadly to men, the basilisk has one fatal flaw: weasels. The smell of the small mustelid is lethal to the basilisk, and men hunt them by dropping weasels into their lairs.
There is little allegorical interpretation of the basilisk, and most bestiaries simply use the creature as evidence for God’s benevolence. Though He has (for reasons best-known to Himself) created a repulsive and evil creature, He has also supplied the remedy, in the form of the weasel. There is a hint, too, of the battle between Christ and Satan in the Gospel of Nicodemus, for the devil is overcome by the meek and humble Son of God, just as the mighty basilisk is defeated by the tiny weasel. Snakes, of course, have been beasts of Satan since the book of Genesis.