Term for competitor that is not the favourite

Competitors below the favourite or favourites are the field. Least-favoured competitors, for whom long odds are quoted by bookmakers, are long shots, and a winner who was given little or no chance of winning is a dark horse.

If one competitor is the favorite, the other is the underdog.

Sticking with betting terminology you could also say "outsider" if the odds against are high.

Or "less fancied" runner.

First things to come to mind are cannon fodder and also rans.

Also rans implies the race is over, but I've heard it used to describe competitors that are not expected to place.

Cannon fodder is probably a bit more mean spirited, but gives a sense of providing enough members in the race, but of lesser quality.