Do Iguanas See at Night?
Iguanas have excellent vision and can distinguish between hues and ultraviolet light. They also have a unique photosensitive organ called the pineal gland, or parietal eye, on top of their skulls. This enables them to see in dim light, making them great night hunters.
Billy has also appeared in films other than Saw. Insidious, Dead Silence, and Death Sentence all featured him as an Easter egg.