basilisk (n.)

fabulous lizard-like creature, c. 1400, earlier basiliscus (Trevisa, late 14c.), from Latin basiliscus, from Greek basiliskos name of a kind of serpent, also the golden-crested wren, literally "little king," diminutive of basileus "king" (see Basil); said by Pliny to have been so called because of a crest or spot on its head resembling a crown.

The basilisk has since the fourteenth century been confused with the Cockatrice, and the subject is now a complicated one. [T.H. White, "The Bestiary. A Book of Beasts," 1954]

Its breath and glance were said to be fatal. The South American lizard was so called (1813) because it, like the mythical beast, has a crest. From 1540s as a type of large cannon throwing shot of 200 lb.