cockatrice: [14] The name of the cockatrice, a mythical serpent whose glance could kill, has a bizarre history. It started life as medieval Latin calcātrix, which meant literally ‘tracker, hunter’ (it was formed from the verb calcāre ‘tread, track’, a derivative of calx ‘heel’). This was a direct translation of Greek ikhneúmōn (a derivative of ikhneúein ‘track’), a name given to a mysterious Egyptian creature in ancient times which was said to prey on crocodiles.
At one point Latin calcātrix, later caucātrix, came to be used for the crocodile itself, but this application never gained much currency in English (which adopted the word via Old French cocatris). Instead, it was adopted as another name for the basilisk, a mythical serpent. The accidental similarity of the first syllable to cock led both to the embroidering of the basilisk/cockatrice legend, so that it was said to have been born from a cock’s egg, and to the word’s 16th-century rerouting as a heraldic term for a beast with the head, wings, and body of a cock and the tail of a serpent.
cockatrice (n.)
late 14c., from Old French cocatriz, altered (by influence of coq) from Late Latin *calcatrix, from Latin calcare "to tread" (from calx (1) "heel"), as translation of Greek ikhneumon, literally "tracker, tracer."
In classical writings, an Egyptian animal of some sort, the mortal enemy of the crocodile, which it tracks down and kills. This vague sense became hopelessly confused in the Christian West, and in England the word ended up applied to the equivalent of the basilisk. A serpent hatched from a cock's egg, it was fabled to kill by its glance and could be slain only by tricking it into seeing its own reflection. Belief in them persisted even among the educated because the word was used in the KJV several times to translate a Hebrew word for "serpent." In heraldry, a beast half cock, half serpent.