Though it is a common characteristic of many animals, one of the first things the boy associates with horses that is mostly a human construct is the idea of their beauty. Throughout the story he notes how attracted he is to horses, that just seeing them brings him pleasure.
He goes on to say "There isn't anything so lovely and clean and full of
spunk and honest and everything as some race horses." The idea of horses being clean and innocent and without ulterior motives is another attractive thing about them to the boy. This is contrasted with the sometimes dishonest and dirty nature of the human beings he associates with.
He also notes that they are "full of spunk," that they are spirited and always ready to compete and try.
The boy also goes on to describe some of the horses in the way a lovesick boy might describe the girl he yearns for. They have unseen and almost indescribable qualities that make you love them and fill you with pain at their separation.