I'm mostly looking for fighters who relied on speed and athleticism, then declined rapidly when those faded. I know RJJ and Sergio Martinez are two.
Those type of fighters don't usually take a lot of punishment and don't get shot 'overnight.' RJJ wasn't really shot all at once. Two fighters who went very good form to shot in very fast time were Matthew Saad Muhammad and Cornelius Boza-Edwards.
Rid**** Bowe beats everybody for brevity Aaron Pryor can also be proposed as the prototypical short prime (a great prime anyway) ... I would even dare including Joe Frazier (at to a point)