Final answer:
The Houyhnhnm master finds the treatment of horses revolting due to the cruelty and inhumanity involved.
Step-by-step explanation:
In Gulliver's world, the Houyhnhnm master finds the treatment of horses revolting because it represents cruelty and inhumanity.
The horses are subjected to mistreatment and abuse, causing the Houyhnhnm master to feel disgust. This treatment goes against the principles of compassion and respect for all creatures.