Hi again people of the fandom, ClassicFan92 here. Now, after last night's episode of The Flash, we learn that Zoom is in fact Jay Garrick, an alias used by the criminally insane Earth-2 resident, Hunter Zolomon. After hearing his backstory, I've thought up of two canidates for the prisoner .
Theory 1: James Zolomon : The father of Zoom, and a vetrean of the War of the Americas, who violently killed Hunter's mother in front of young Hunter. He has him captive to both torture and humiliate him. When creating the Jay Garrick persona, Hunter probably took James' name in revenge, and shortened it.
Theory 2: Ronnie Raymond : As it turned out, he survived closing the singularity, only to wind up on Earth-2, where he was captured by Zoom and tortured for information about the Flash of his world. Since Zoom time traveled to create the illusion of Jay Garrick, it's likely Ronnie was unaware the two were the same person until King Shark. Unlike Martin Stein, and like the comics, Ronnie doesn't need to be fused with somebody in order to become Firestorm after awhile. This would explain why he didn't have the same issue Martin did during the Fury of Firestorm episode.
Now keep in mind, these are just theories, nothing concrete.