Horse sense? Or Zebra sense?

In Guns, Germs and Steel, Jared Diamond asks why of 148 wild herbivorous mammals that could be considered candidates for domestication by humans, only 14 passed the test (or failed, depending on whose perspective you take I suppose). In answering this question, he uses what is known as the Anna Karenina principle (deficiency in any one of a number of factors results in failure). The 6 factors used are:

• Diet,
  
• Growth rate,
  
• Problems of Captive breeding,
  
• Nasty Disposition,
  
• Tendency to Panic, and
  
• Social Structure,

Diamond points out that the Asiatic ass (pictured), also known as an Onager, has a nasty disposition towards humans and that "All writers about them, from Romans to modern zookeepers, decry their irascible temper and their nasty habit of biting people".

Diamond doesn't explore why it might be that Onagers are so nasty to humans, but my suspicion is that they're extremely clever animals!

Likewise with the Zebra:

Zebras have the unpleasant habit of biting people and not letting go. They thereby injure even more American zookeepers each year than do tigers.

Is it possible that these animals knew the score, that they knew what was at stake and were intelligent enough to deliberately avoid domestication? Or do they just have good instincts?

Zebras are also virtually impossible to lasso with a rope - even for cowboys who win rodeo championships by lassoing horses - because of their unfailing ability to watch the rope noose fly towards them and then duck their head out of the way.