E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The king of serpents (Greek, basileus, a king), supposed to have the power of looking any one dead on whom it fixed its eyes. Hence Dryden makes Clytus say to Alexander, Nay, frown not so; you cannot look me dead. This creature is called a king from having on its head a mitre-shaped crest. Also called a cockatrice, and fabulously alleged to be hatched by a serpent from a cocks egg.