Emile Griffith and Bennie Paret @ 1961. (Dates a little off, but it’s close) I’ve seen this one. It’s absolutely one of the most horrifying sequences I’ve ever seen–one you don’t want to see again. Paret was trapped against the ropes, out on his feet, and unable to fall, absorbing one after another hard punch from Griffith, each punch snapping his unconsious head round like a punching bag. Referee Ruby Goldstein, one of the most experienced referees in fight history, inexplicably froze, and did not step in until it was too late. IIRC, that was the last fight he ever refereed.
Rocky Marciano and Carmine Vingo @ 1949 in Madison Square Garden. As far as I know, no footage exists of this fight, but we have the descriptions of what happened. Vingo was also out on his feet, and taking a merciless pounding from Marciano. When he fell to the canvas, it was obvious that there were serious problems. He was carted off to a nearby hospital, where he was not expected to make it. He did survive, however, although that was the end of his boxing career. In fact, he was alive as recently as two years ago, but he might have died since then. Ironically, Marciano, the victor in that bloodbath, was the one who would have the much shorter life.
Rocky Marciano and Roland LaStarza (II) @ 1953. In their prior meeting, which I think was immediatly after the Vingo fight above, Marciano eked out a split decision victory. In this one, he didn’t want to take any chances. LaStarza covered up (sort of a rope-a-dope), but Marciano didn’t care–he hit whatever was available, in this case, LaStarza’s forearms. By the 11th round, LaStarza’s arms had been battered so badly he could not continue, and needed surgery to correct the damage to the blood vessels in his arm.
Some other well-known fighters who had opponents who died as the result of ring injuries: Ezzard Charles (as a middleweight or light-heavy), Sugar Ray Robinson, Max Baer, and Primo Carnera.
None of these might be what you’re looking for, but they’re the obvious ones that come to mind.