"I mentioned it a few times because after 'Rocky II' came out and made a ton of money and then 'Rocky III' hit and made more than all of them, I said I'd like to have some ownership since I invented it," he said. "And that never happened. So I have zero ownership of Rocky."
The actor claimed he approached his attorney Jake Bloom (who declined to comment on the story) but was shot down, and didn't bother going to his agents at CAA as a result.
"I was told it just doesn't happen, no one has it, and look, they're giving you more money on 'Rocky III.' They took their chances, and you're not entitled to it," he recalled.
"I think there was a certain code of business conduct, maybe not as much now, but back then, that you don't ruffle the feathers of the golden goose. The studio is the power, the agency relies upon them, and the attorneys are the go-betweens."
He said that when he did finally confront them, just before the release of Rocky IV in 1985, he said: "'Does it bother you guys that I've written every word, I've choreographed it, I've been loyal to you, I've promoted it, directed it and I don't have 1% that I could leave for my children?' And the quote was, 'You got paid.' And that was the end of the conversation."
Stallone claimed he was told the money made off 100 different Rocky licences and merchandising deals worth "hundreds of millions of dollars over 45 years" would be put into a pot — but he never saw a cent of it.
"I was very angry. I was furious. 'Rocky' is on TV around the world more than any other Oscar-winning film other than 'Godfather.' You have six of them, and now you have 'Creed' and 'Creed II'."
Of course, Stallone did make plenty of money from the film series. The first film made $225million off its meager $1million budget, a success story equitable to its protagonist. Stallone famously went from unknown struggling actor forced to sell his beloved pet dog to make rent, to household name practically overnight. After the studio bought the script they had a line-up of A-listers in mind for the lead, but Stallone doggedly fought to play it himself, winning the right against the odds.
He went on to receive Oscar nominations for Best Actor and Best Original Screenplay, becoming only the third person in history to be nominated twice for the same film, after Charlie Chaplin and Orson Welles.
"I love the system — don't get me wrong. My kids and their kids, they're taken care of because of the system," Stallone added. "But there are dark little segues and people that have put it to ya. They say the definition of Hollywood is someone who stabs you in the chest. They don't even hide it."
After appearing in a number of flops around the turn of the century, Sly's career was in a downturn — so much so that he was even dropped by his agents, which left him in an even weaker position to negotiate ownership.
"I never really pushed it, and by the time we got around to 'Rocky Balboa' I was in a pretty weak position to say anything. I was in a slump, and it was pretty intense. There was a great sense of finality about it. I couldn't get arrested," he said. "I was let go by my agency. Dropped — fired basically."
But he turned things around with the sixth installment of the franchise in 2006, which saw the character come out of retirement for one last fight.
"There was such skepticism, and rightly so. It's like doing 'Godfather 20.' I get it," he said. "The core audience had grown up and was gone. I said it's not about boxing — it's about grief and loss. He finally lost his love, which is the end of the equation for him. The only way he knows how to deal with it is cathartically through fighting. He says, 'I got this beast inside,' and it worked."
"I had seen this town from every peak, and I'd seen it from the valley. I know the terrain pretty well, and the fact that Rocky [Balboa] prevailed one more time was even more momentous for me than 'Rocky I.' This was tough. There was no element of surprise, you're well past the right age, and the audience that came — my demographic didn't show up at all — it was 28- to 30-year-olds."
After Balboa came the critically acclaimed and financially successful Creed spin-offs, which saw Balboa mentor his old friend and rival Apollo Creed's son Adonis (Michael B Jordan), and also earned Sly his third Oscar nom, for Best Supporting Actor.
As for the future of the franchise, he revealed there may be a prequel TV series in the works, and another Rocky film in which he takes an undocumented street fighter under his wing.