I'll echo Sohcahtoa's sibling comment to yours, but with a different pair of species: horses and donkeys (mules count as donkeys for this one). Donkeys have a

I'll echo Sohcahtoa's sibling comment to yours, but with a different pair of species: horses and donkeys (mules count as donkeys for this one). Donkeys have a reputation for being stupid and stubborn because they're smarter than horses. Too independent for easy training, and will refuse an idiotic command.