Pony is least close to Steve, Soda's best friend. Steve thinks of Pony as a child, and doesn't like the fact that Soda is always bringing Pony along.
This, of course, really bugs Pony, but he's not interested in causing trouble between Steve and Soda. Plus, Steve is part of the gang and so Pony is loyal to him, at least in that context. Like Two-Bit, Steve doesn't undergo changes, do anything to drive the plot, or act in a way that influences the story's outcome.