Why was Gulliver’s master convinced that Gulliver must be a Yahoo, but what perplexed him about it?

The horses believe that Gulliver is a Yahoo-but a more rational and civilized Yahoo. Gulliver certainly looks like the naked Yahoos in the stable, yet possesses a keen but flawed intellect. Gulliver, wanting to separate himself from the Yahoos as much as possible, asks not to be called a Yahoo anymore.