There has been many theories of who Zoom is but here is mine!
He might be the reverse flash of earth 2!
Seeing that Barry Allen had a main antagonist in the reverse flash it might be that way for Jay Garrick.
There is no hard evidence to back this up but we haven't seen the reverse flash of earth 2. It's all just speculation! But they both looked like at times horrible animalistic creatures when they vibrate their molecules.
Also Zoom kinda looks like the new 52 reverse flash but blue!
This is a bit of a long shot but still pretty good!