The basilisk is a creature which actually holds two distinct descriptions. The traditional depiction of a basilisk is akin to a cockatrice, being part cockerel, part dragon, and possesses a killer stare. A more modern depiction is that of a giant serpent with the ability to petrify any who meet its gaze. Both incarnations are referred to as the King of Serpents, and both are also mortal enemies of the weasel, who is for some reason immune to the powers of the chimerical beast.
Ancient accounts of the the basilisk tend to generally match, but the size of the creature varies from very small (roughly the size of a rooster) to very large (larger than a horse, perhaps the size of a dragon). Generally, it is described as having the head and legs of a rooster and the body, wings, and sometimes the tail of a dragon. Its breath is venomous and its stare can kill. An indirect glance, however, turns its victim to stone.
The basilisk is said to be born from either a snake or toads egg hatched under a cockerel, the very opposite of its very close relative, the cockatrice.
It is possible that the original description of the basilisk came from time-twisted eye-witness accounts of the King Cobra, which tends to have a vaguely crown-shaped marking on its head, as well as being the natural enemy of the mongoose (which itself was mistaken for the weasel).