Yeah, it was mainly Chuck Wepner. An unknown Stallone saw a real underdog being given a shot at the The Greatest Prize in All Sports, as the World Heavyweight Title used to be known. Wepner's big moment was knocking down the Champ, Ali. It was a balance shot, Ali wasn't hurt one bit... but still, Wepner threw the shot and Ali went down from it, so it counts!

Smokin' Joe Frazier had a lot to do with the Rocky character too.
Joe was from the south, but moved to Philadelphia as a teenager.
Not a south paw like Rocky, but Joe's punch was his Philadelphia left hook.
Rocky had the right hook.
Joe worked at a slaughterhouse, and would punch the sides of beef.
Joe also jogged the steps at the Philadelphia museum.