Pony is an outsider because he is a greaser, and he is also different from the other greasers.
An outsider is someone who does not fit in. Ponyboy Curtis is twice an outsider. First, all of the greasers are outsiders. They are poor, and they wear their hair differently. Because of this, they are looked down upon by mainstream society. However, even among the greasers Pony is an outsider because he is different.
Pony is reflective and introspective, and very intelligent. Unlike many greasers who have dropped out of school, Pony likes school and gets good grades. He loves reading and going to the movies, both as intellectual pursuits. Pony stands out among his friends because he is not quite like them.
And nobody in our gang digs movies and books the way I do. For a while there, I thought I was the only person in the world that did. So I loned it. (Ch. 1)
Pony abhors violence, and is in fact not very good at fighting. He takes part in rumbles only rarely, such as when he wanted to fight the Socs over Johnny. Pony prefers to have his nose in a book.
Unlike most of the other greasers, Pony has a future. He has a chance to not be a greaser someday, through education and hard work. It is also significant that he does not want to be like the others, but he identifies with them too. They are still his friends, and where you come from matters.
Finally, Pony is able to see beyond class and gang conflicts. He notices that Cherry is not like he expected a Soc to be. He also has an honest conversation with Randy about violence and the clash between the gangs. Pony is able to realize that Socs and greasers do have a lot in common.