Mongolians gained a lot of experience how to choose fast horses, while they were breeding them. 
There are many features to recognize  fast horses.
They are: the origin, anatomy, stride, sound produced by a horse  ets.
A  horse containing all these features together should  have be the fastest horse. The  importance Mongolians gave in fast horses  is their origin and outlooking.Usually fast horse looks like  other fast wild animals , like : whild ass, antelope, deer ets.
As written in the ancient books there are 40 kinds of different features to recognize fast horses.Let`s see one example: The dragon horse  has wide forehead, wide eyes, straight back, big skull, long neck. The horse contains these qualities can jump very fast even in the rain.The fast horse produces thin, loud sound as a lark. A fast horse has wide chest , long ears and big nostril.
  (By Davaahuu .D  1999)