In chapter 17, Holden is on a date with Sally. He is trying to tell her how shallow people really are. He has seen some boys he knows from another prep school, and he is angry at the attitudes of most people.
He thinks people who are crazy about cars, just care about showing off in front of other people. He says that they buy new cars and immediately start thinking about trading it in for an even newer model. He thinks they don't care about anything important, just looking good in front of others. He becomes angry at how society is making people just care about looking good. He thinks people should just go back to owning horses. He says that a horse is the complete opposite. Holden says that horses are simple and honest. He even says they are least human.
Holden's points about the two are symbols on how Holden is trying to stay in the past. He is trying to relive his childhood in a way. It was an innocent time. His comments about owning a horse, is just another way he is living in the past. What little boy doesn't want to own a horse? We can see that Holden's mind is not functioning the way it should. He constantly keeps thinking of things from his past. He wants to stay as a child and not face the realities of becoming an adult.