Regulus is one of the fifteen Behenian stars (stars that are specifically useful for magical application in the medieval times)  On this list is Sirius and Arcturus (is that name important, i can't remember)

The basilisk is usually described as a crested snake, and sometimes as a #### with a snake's tail. It is called the king (regulus) of the serpents because its Greek name basiliscus means "little king"; its odor is said to kill snakes. Fire coming from the basilisk's mouth kills birds, and its glance will kill a man. It can kill by hissing, which is why it is also called the sibilus. Like the scorpion it likes dry places; its bite causes the victim to become hydrophobic. A basilisk is hatched from a ####'s egg, a rare occurence. Only the weasel can kill a basilisk.

Some manuscripts have separate entries and/or illustrations for the basilisk and the regulus, possibly because the basilisk account in Isidore has three sections, one each for the basilisk, the "kinglet" (reguli), and the sibilus. Where the regulus is treated separately, the bite of the basilisk causing hydrophobia is generally ascribed to the regulus.