What comes in my mind immediately are McLellan beatin' Jackson but accumulatin' so much damage that led to his tragic demise by meeting Benn or Alì firstly at Manilla when him and Frazier beat the crap of their primes outta of each other than defeating Shavers by absorbing an incredible amount of punishment that deteriorated the man's health. Any other you would consider like this? Not necessarily related to this kind of problems obv.
Maybe David Izon(ritei) over Derrick Jefferson? Izon endured a horrific beating throughout a fight that arguably could've been stopped at certain points, only for Jefferson to gas & get stopped in the last round. In Izon's next big fight, he was bombed out easily by Fres Oquendo & quickly descended into stepping stone status.
Frazier in the FOTC--perhaps the ultimate pyrrhic victory. He declined sharply from that point and never again approached his 1971 level of ability and conditioning.
Hearns and Mugabi wins took a lot out of Marvin Hagler. Though he only had one fight after, he seemed slower by the time he faced Leonard. Conversely, Hagler ruined Mugabi, though he didn't beat Hagler, he gave him pure hell. After that fight Mugabi was never the same. Aaron Pryor after the Arguello wins.... Pryor took some savage head shots from Arguello. Along with his personal demons with drugs he wasn't the same.
Robert Quiroga's win over Kid Akeem. Quiroga was just 21 years old at the time, and had only 4 fights following the Akeem fight, going 2-2.
Leonard against Hearns too, didn't Leonard first retire because the jabs he took in his eye and proceeded bingin' between booze, yayo and hooas? He definitely lost his prime right after
It's a reference to the ancient military leader Pyrrhus, who won a battle vs. the Roman Empire, but it came at such great cost to his army that it brought about his downfall.
History nerd here. The Greek general Pyrrhus famously fought three battles against the Romans. Each battle was a brutal affair that bled his army white. Even though he won every battle the cost was so high that it wasn't worth it. He was quoted as saying "One more victory over the Romans and I am undone." Hence, a Pyrrhic victory - one that isn't worth the cost.