I dont bet so dont ever know the odds. Id wager joe louis wasnt an under dog til he had to come back as an old man. Jim Jeffries wasnt ever likely an under dog even in his fight with jack johnson. Wlad, even though maybe dont consider him an atg, which he is..I dont think has been an under dog.
I think it's Mayweather Jr and Hagler. Tyson first time became an underdog was against Lewis. De la hoya first time as an underdog was against Hopkins. Trinidad first time became an underdog was against De la hoya i think.
In regards to Mayweather, the odds for both Corrales and Chicanito were very close. You can probably find sources where the odds have Floyd as either a slight underdog or slight favorite.