I remember reading in the old Boxing Monthly Mag about Robinson coming in late as a replacement to fight John Davidson (Remember him?). He beat him and went on to make 7 successful defense s. The Cinderella man of the time mate.
Many people felt he did beat him in the first fight that's why the demand for the rematch was so high.
I believe Pascal was the 7-5 fave over him as well. Virtually everyone I've seen that scored it had it for Hopkins.
A modern example would be Chris Billam Smith. Everyone saw him as never being able to surpass British level when he was on the way up, everyone picked him against Okolie, he was the underdog against Riakporhe in the rematch, and in general on the way up, no one really thought he could get to the level he did.
I think Wlad gets a shout here. He didn't seem to get written off when he lost to Puritty, but when he lost to Sanders it seemed he wasn't likely to become champ. It seemed like he was almost going to quit boxing when he lost to Brewster. Then he becomes one of the longest reigning HW champs of all-time. Pretty remarkable.
Carl Froch probably deserves a mention. I think Taylor, Abraham and Bute were all Favorites against him, going in.
Seeing Benn and Eubank mentioned, I’ll throw Steve Collins into the discussion. He was a bridesmaid until he hit around age 30, having lost decisions to Mike McCallum, Sumbu Kalumbay and Reggie Johnson (the latter two being split and majority) … then proceeded to knock of Chris Pyatt (whom I assume would have been favored?) before twice besting Eubank and then doubling down with two wins over Nigel.