Basilisk

Mike Dash notes various qualities attributed the basilisk, described in one late 16th century Polish account as having the head of a cock, the eyes of a toad, a crest like a crown, a warty and scaly skin “covered all over with the hue of venomous animals,” and a curved tail, bent over behind its body. On the one hand the beast was highly toxic:

The bodies were pulled out of the cellar with long poles that had iron hooks at the end, and Benedictus examined them closely. They presented a horrid appearance, being swollen like drums and with much-discoloured skin; the eyes “protruded from the sockets like the halves of hen’s eggs.” Benedictus, who had seen many things during his fifty years as a physician, at once pronounced the state of the corpses an infallible sign that they had been poisoned by a basilisk. When asked by the desperate senators how such a formidable beast could be destroyed, the knowledgeable old physician recommended that a man descend into the cellar to seize the basilisk with a rake and bring it out into the light. To protect his own life, this man had to wear a dress of leather, furnished with a covering of mirrors, facing in all directions.

 On the other hand it could be used in the creation of gold:   

basilisk powder, a substance supposedly made from the ground carcass of the king of snakes, was greatly coveted by alchemists, who... believed it was possible to make “Spanish gold” by treating copper with a mix of human blood, vinegar and the stuff.