Which real-life boxer made a cameo in the first "Rocky"?

I know what you’re thinking: "Rocky IV" is the best of the series. But alas, we can’t ever get to that one without talking about the one that started it all, 1976’s "Rocky."

The iconic boxing movie has a notably more artsy style and slow pace to it compared to the sequels. But I guess that worked for people in 1976, as it went on to be nominated for 10 Academy Awards and won three, including Best Picture. 

It was also no slouch at the box office, pulling in $225 million against just an $800,000 budget. 

The realism of the movie even included a cameo appearance from a real-life boxing great. Who was it?

Click START below to answer.