You're not really very clear about precisely what you mean by who was Pony Boy's surprise visitor. Dally surprised them by visiting at the cabin, and that is how Dally happened to be there to help Johnny and Pony Boy rescue the school children from the burning church.
Pony Boy was also surprised when a car full of Soc's showed up at the Tasty Freeze and he realizes that these are the very same Soc's who beat them up the night Johnny killed Bob. Pony Boy's reaction is deep and intense. The hatred he feels for them burns in his gut. Randy confesses that he doesn't want to fight (and won't) in the rumble. Add to this Randy's confession about Bob, and Pony Boy comes to the realization that Socs and Greasers aren't so different. Both are human with human emotions and human frailties.