Subject: What is the difference between horses, donkeys and zebras besides the skin?

A horse, zebra and donkey look very similar. The apparent difference 
between them is their height and colour of their skin, and the hair 
on their neck. In one question/answer posted on cross-breeding, it 
states that horses are different from donkeys. What are the any 
physiological difference between them? Are the difference between 
these three animals same as difference between humans & monkeys 
(without tails), or difference is like whites (Europeans), blacks 
(africans) and east asians (chinese, japanese, koreans, etc)?

Re: What is the difference between horses, donkeys and zebras besides the skin?