Depending on who tells the tale, the basilisk can be portrayed as a venom-spitting serpent, a ferocious lizard, a towering dragon, or a full-fledged chimera sporting the head and wings of a rooster attached to the body of a snake. Like the gigantic green serpent that Harry encounters in the Chamber of Secrets, the basilisk is always frightening and often deadly, having the power to kill its victims by its stare alone.

We first hear of this reptile from Pliny the Elder, the first century latin writer whose book, Natural History reflects much of what ancient Romans believed about the natural world. According to Pliny, the basilisk is a small but letal snake, no more than 12 inches long, and native to North Africa. Known as "The King of Serpents" because of the crown-like markings on its head (basiliskos means "little king" in Greek) the basilisk advances on its prey with its body held upright rather than wriggling across the ground like other snakes, and could set fire to bushes and break stones apart simply by breathing on them. The basilisk lived in the desert, not necessarily because it chose to, but because no matter where it lived, the land was eventually reduced to to deserts by its scorching breath. Its venom was so powerful, Pliny reported, that if a rider on horseback killed a basilisk with a spear, the venom would rise up through the spear, kill the rider, and then kill the horse as well.

Basilisks cannot tolerate the scent of a weasel or the sound of a rooster crowing. To slay a basilisk by weasel, it is necessary to lure the basilisk into a weasel's den and then block the entrance and exit, whereupon the basilisk would succumb to the weasel's fumes. Death by rooster is far easier as, according to the Roman writer Claudius Aelian, the mere sound of a rooster crowing would cause the basilisk to go into convulsions and die (that's why Tom Riddle arranged for the deaths of so many roosters in Chamber of Secrets). But, perhaps the best defense of all is to hold a mirror up to the basilisk, turning its own fatal gaze against it and causing it to die of fright.

Like many imaginary creatures, the basilisk was probably based on a real animal - in this case, the Egyptian cobra, which has letal venom, moves with its head held upright, and has markings on its head resembling a crown

By the Middle Ages, popular books of mythical beasts had begun to describe to describe the basilisk as a bizarre monster having the body of a snake (it was all snake in Greek and Roman traditions) and the head, wings, and sometimes feet of a rooster. This verison of the creature, which legend has it can be found in England as well as in Africa, was known as a cockatrice as well a basilisk. The unlikely combination snake and chicken probably came from stories concerning its birth, which told of a basilisk hatching from a rooster's egg that had been laid on a hillside and incubated by a toad.

The basilisk of the Chamber of Secrets, however, is clearly of the older type, strictly a serpent and a huge one, too, as befits the Heir of Slytherin.