There is no specific symbol associated with Prometheus, but he is most closely associated with fire. Prometheus was the immortal who gave mankind the gift of fire in direct opposition to Zeus' will.
In Greek mythology, Prometheus is one of the TItans, an earlier race of deities that ruled before being overthrown by the Olympian gods. The leader of the Olympians, Zeus, decided to withhold the gift of fire from humanity, but Prometheus hid the fire in a fennel-stalk and gave it to mankind. As punishment, Zeus had Prometheus chained to a rock, where every day his liver was eaten by an eagle. Every night, his liver regenerated, and being immortal, his torment was meant to be eternal. Heracles (better known as Hercules in Latin) found Prometheus, killed the eagle and freed him from his chains.
There are a number of other myths regarding Prometheus. One myth credits Prometheus with shaping the first humans out of clay, giving them their god-like shape. In another story, he tricks Zeus into accepting the bones and fat, rather than the meat, of a sacrificial animal. In all the stories, Prometheus is seen as a benefactor of humanity.
The theft of fire is a common trope in ancient mythology. Similar stories appear in Hebrew mythology, in the Hindu Vedas and in a large number of native American myths. It is reasonable to surmise that fire meant more to ancient people than just a useful tool. Fire also represented the divine spark of wisdom and creativity.