They go by many labels. The polite terms are trial horses, steppingstones, gatekeepers, B-sides. Others would dare to call them palookas, bums, stiffs, pugs, cadavers, cannon fodder though maybe not to their faces.
These are the sort of men, practically anonymous to all but the staunchest fight fans, who compose the early portion of every champion's career. They build up confidence. They build up a record. And then when their usefulness has been exhausted, they usually fade away.