The stories of Heracles take place way back in the day. And we mean waaaaaaaay back in the day. By the time writers started scribbling their versions of Heracles' adventures, the stories of his exploits had already been around for many years.
Some call the era of Heracles and his fellow heroes the Heroic Age, the time when the heroes were all direct descendents of the gods and the world was crawling with monsters that needed slaying. It's important to think about the fact that even to the writers who originally recorded the legends of Heracles, he was a figure from the distant past.