a horse, team, etc., that has little chance of winning and carries long odds.
an attempt or undertaking that offers much but in which there is little chance for success.
Movies, Television .*snip*
That means that by definition, the 2 Lotus powered cars are the quintessential long shot.
Under the definition of long shot, Marty Roth and/or Milka Duno were the poster children for Long Shots to win - a long shot is a hail mary...you bet on them because you'll have a huge reward with a very teeny chance of success.
You're the one who changed the definition to suit your own purposes.
Under dog is
[uhn-der-dawg, -dog] Show IPA
a person who is expected to lose in a contest or conflict.
So, Marty Roth and Milka Duno were both Under Dogs as well. As are the Lotus powered cars.
As a matter of fact, Thesaurus.com has both "under dog" and "long shot" listed as synonyms for dark horse.