Jeff Fenech fits. Went from an undefeated 3 weight champ ( should’ve been an undefeated 4 weight champ) to a potential KO victim every time out subsequently. The Nelson loss fair enough but he also got blasted out by Calvin Grove and destroyed by Phillip Holliday. His fall was very dramatic.
Big John became a coke addict after he won the title from Gerrie Coetzee , Gerrie had a very good right hand and Tate stood up to it well, but after he became addicted it was over. His losses to Weaver and Berbick had more to do with what John was doing and his poor management than his opponents abilities.
How about from more recent times Juan Manuel Lopez ? After Salido beat him up twice and burst his bubble he was never the same.