A strong single answer is the camel : it is widely known for stamina and perseverance because it can travel long distances in harsh desert conditions with very little water.

Other good examples are:

  • Arctic tern for endurance in migration.
  • Pronghorn for sustained running speed.
  • Sled dogs for long effort in extreme cold.

If you want the animal that best matches the idea of “keeps going no matter what,” the camel is the clearest fit.