As defined by Funk & Wagnalls:

underdog n. 1. One who is at a disadvantage in a struggle. 2. One who is victimized or downtrodden by society.

An underdog is anyone who has to face a higher level of adversity to reach a goal (often survival, or "winning") than either their opponent(s), contemporaries, or peers. Often the classic underdog is a victim of an antagonizing opponent. Moreover, a classic underdog typically has positive qualities that are absent in his or her stronger or better armed opponent.