According the the Houyhnhnm master, Gulliver's physical anatomy is sorely lacking in strength and purpose. In chapter four, the master horse and Gulliver enter into a conversation about yahoo physiology. These were the main areas that the master horse found sadly lacking:
1. Fingernails and toenails--completely worthless.
2. Gulliver's 'forelegs' (his arms)-- He cannot walk on them, so what good are they?
3. The arrangement of the face-- The yahoo face was much too flact, not allowing him to look from side to side without turning his head. Also, yahoos cannot feed themselves with their mouths alone, but must assist the food into the mouth via the hand.
4. Impractical feet--The general shape and softness of the feet do not allow for travel over hard terrain.
5. Natural repugnance-- Every other creature in the land seems to abhor the yahoos.