I would say Pony is a hero in two ways. He is a hero in the conventional sense for helping to save the kids and he is a hero in a less conventional sense for overcoming his problems.
We usually think of heroes as people who rescue other people or do something equally brave. Pony did this, and so he is a hero in that way.
But to me, a hero is also just someone we can or should admire. And I think we should admire Pony for overcoming all the problems he has and the temptations to become a bad person. I think Pony is admirable because he is going to "stay gold."