"swift has intercharged the roles of humans and horses in country of houynhnms"what satiricals propose did jonathan sfift wants to achive through this?

Gulliver uses a universally common "beast of burden" to elevate them to a sense of greater enlightenment than humans. Horses had been domesticated servants to man for centuries. Swift is apt to make the point that horses were not just subservient to European powers but also other cultures as well. Swift hence makes a wider statement about human nature rather than just European hypocrisy and absurdity.

The houynhnms surpass humans in every moral and intellectual level. These subservient beasts in Europe rule over the inferior humans,

"[I] wondered more to see the rest employed in domestic business... However, this confirmed my first opinion, that a people who could so far civilise brute animals, must needs excel in wisdom all the nations of the world."