Sylvester Stallone wrote the screenplay for "Rocky in three and a half days, shortly after watching the championship match between Muhammad Ali and Chuck Wepner that took place at Richfield Coliseum in Richfield, Ohio on March 24, 1975.

Wepner was TKO'd in the 15th round of the match by Ali, but nobody ever expected him to last as long as he did.

Although other boxers, including Rocky Marciano added flesh to the movie and the sequels, it was the stamina of Chuck Wepner who inspired "Rocky" to begin with.

Muhammad Ali, perhaps the greatest boxer in history, passed away in Scottsdale, Arizona on June 3, 2016 from "septic shock." He was an inspiration to Stallone, and the two had a wonderful friendship.

Rocky Marciano

He was one of the greatest Heavyweights in the history of boxing, and even resembled Stallone in his part as Rocky.

James J. Braddock, also known as the "Cinderella Man".

Despite being a 10-1 underdog, on June 13, 1935, at Madison Square Garden Bowl, Braddock won the Heavyweight Championship of the World in what was called "the greatest fistic (sic) upset since the defeat of John L. Sullivan by Jim Corbett". This would be "Rocky IV".

More Info: en.wikipedia.org