It's hard to describe how wildly popular Heracles was back in the day, but we'll try. Imagine everybody you know who likes Harry Potter. Got it? Okay, now imagine everyone you know who doesn't like Harry Potter. All right. Now imagine that all of those Potter naysayers suddenly adore the boy wizard. Hmm, that's a lot of people, right? Of course, we've got to take it one more step to push it to Herculean levels: imagine that all those people literally worship Harry Potter as a god. There you go. That's how much people liked Heracles. We're not exaggerating either. There really were shrines to Heracles all over the ancient world, where he was worshiped as a champion and protector of mankind.
But was the most famous hero-turned-god ever a real person? No one really knows. Some say that he was a Greek king long ago who probably did some pretty awesome things. Then when he died, the stories about him got more and more exaggerated until he became the superhuman Heracles that we know today. On the other hand, some say that he was never a real person at all, but instead the character was a combination of a lot of local heroes from all over the place whose legends eventually crystallized in the form of the greatest Greek hero of all time.
Wherever Heracles came from, it's clear from the earliest literature we have that his legend has been around for a while. He pops up in Homer's Iliad, his ghost makes a cameo in the Odyssey, and he's talked about in Hesiod's Shield of Heracles and Theogony. He also pops up in two famous extant tragedies: Heracles by Euripides, and The Women of Trachis by Sophocles. Later on, several of his stories were told by the Roman poet Ovid in his epic poem the Metamorphoses.
Heracles' popularity continued on through the centuries and hasn't died down yet. We might not worship him as a god anymore, but most recently the great hero got his own Disney movie, a long running TV series, and his own comic book from Marvel. Not too shabby for somebody who might never have existed at all.