Super Saiyan was the most popular legend of the Saiyan race. The last Saiyan to become a Super Saiyan had one thousand years prior. The legend had become so prevalent that even has Freeza heard of the legend from the conversations of the Saiyns. The fear of a Saiyan becoming a Super Saiyan triggered Freeza to destroy Planet Vegeta. Years later, Son Goku fulfilled the legend on Namek and transformed into a Super Saiyan in a battle against the evil emperor, Freeza.