An also-ran is a term to describe someone who lost a race, either athletic or political, by a large margin, or someone of little significance. It is always hyphenated. The plural is also-rans
Its synonyms include nonstarter and loser. The thesaurus we referenced listed dark horse as a similar term, however, a dark horse or sleeper is an unsuspected victor, not failure.
As you can see by the ngram below, also-ran has had a dramatic increase in popularity since its coinage at the turn of the twentieth century. It was first used to describe horses who also ran in a race and did not place.