Participant in a competition that is expected to lose
An underdog is a person or group in a competition, usually in sports and creative works, who is largely expected to lose. The party, team, or individual expected to win is called the favorite or top dog. In the case where an underdog wins, the outcome is an upset. An "underdog bet" is a bet on the underdog or outsider for which the odds are generally higher. The first recorded uses of the term occurred in the second half of the 19th century; its first meaning was "the beaten dog in a fight". In British and American culture, underdogs are highly regarded.
https://en.wikipedia.org/wiki/Underdog