Why did Swift make the houyhnhnms look like horses in part 4?

In this section of the ,
Swift is, in part, poking fun at the entirety of the human race and our perception of our own
intelligence and feelings of mastery over other animals or even, sometimes, other people.  The
horse is a long-domesticated beast of burden that many of Swift's contemporaries felt to be
beneath them in terms of intelligence, and so he presents a utopian society in which horses rule
in order to turn this common way of thinking on its head.  Unlike human society, the society set
up by the Houyhnhnms is peaceful and reasonable, and it is actually the Yahoos, the horribly
disgusting and animalistic humanoid creatures, who are the beasts of burden for the Houyhnhnms. 
By presenting humans and horses in these ways, Swift encourages readers to reevaluate the way
they see themselves and other animals and/or people to see if their perceptions are accurate or
merely the common way of thinking.