Early Basilisks were described as small snakes with a crest on the head like a crown (from the Greek "basilieus" meaning king, as in "king of snakes".) The basilisk was extremely poisonous and even its breath or glare could be fatal.In Heraldry, it is mostly the same as a Cockatrice, sometimes differentiated by an additional head (often a dragon) at the end of the tail.
King of Serpents - Amphysian Cockatrice
The cockatrice was originally described as taking the form of the basilisk, but by the 1400's the name had morphed from basilisk to basilicok to cockatrice, helped by a mention in Chaucer's Canterbury Tales. This is probably where it picked up the partial attributes of a Cock, and became a different creature. In the same manner, the Basilisk originally referred to a small snake with a crest on its head (like a crown, hence the title "king") Of course, as time went on and the stories got more exaggerated, the snake got bigger and deadlier. In the Middle Ages, legends told that the basilisk could only be killed by a weasel or a cockerel, and many travelers carried these other animals in case they encountered a basilisk.
In Christian art, it is the emblem of sin and the spirit of evil. The image above is often used in heraldry, even though it is different than many of the descriptions.