The basilisk is usually described as a crested snake, and sometimes as a cock 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 by a toad from a cock's egg, a rare occurrence. 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.

Basilisk illustrations vary greatly. Most show a bird-snake hybrid, with the beast being a bird (often a cock) with a snake tail. In some images the basilisk is all snake with no bird parts. The basilisk as king of the serpents appears in some manuscripts, with the basilisk wearing a crown. The weasel attacking the basilisk is commonly included in the image.