Zoom's identity is something that is shrouded in mystery. Here are a multitude of possibilities as to who he may be:

Henry Allenas we've seen from Enter Zoom, the Green Arrow from Earth-2 is Robert Queen. With Henry Allen finally being released from prison, have we seen the last of him? Robert Queen being Green Arrow is enteriing Flashpoint (comic) territory. Zoom could be a corrupted version of Henry Allen. But from what we have seen from the episode, we did not see a reaction from the maniacal speed-obessesed villain. 

Wally West: this is a bit expected and speculated primarily based on the fact that they have casted Tony Todd for Zoom. But they had confirmed that Tony Todd is simply doing the voice of Zoom. Wally West is expected to come up some time in the near future. We don't know jack about Wally but we could see his past being much like that of the New 52; Wally being a troubled youth. 

Barry Allen: its a fairly common element to happen but its a distinct possibilty. The biggest threat that Barry has to face is none other than himself. Again, we've seen some hinted Flashpoint areas in Earth-Two. Not to mention how when Barry was having  visions as he went through the speed force; we seem him in jail. Reverse-Flash mentioned how he was seeing the past, present and future all at once; but does that necessarily mean HIS past, present and future? Or was it the future of his double?

Eddie Thawne: Frankly, when I saw Eddie go into the singularity and then saw Zoom's blue lightning in the teaser, my head immediately went to the idea: Eddie's the new big bad. Unfortunately, that isn't the case? Or is it? We could be dealing with some Flashpoint-like stuff. Eddie is law-abiding character who wanted to be seen as a hero. Zoom mentioned that he does not want to be a hero as "heroes die". Maybe before he went on his "speed vendetta", Zoom did try to be a hero and fail? Could this attributed to the apparent Flash/Rerverse-Flash rivalry we have seen in the comics? Not to mention, it could create a massive conflict within Barry as he feels immense guilt that Eddie was the one who made the ultimate sacrifice in killing Eobard. 

Malcolm Thawne, Edward Clariss or Johnny Quick: For those of you who don't know these characters, I'll start with the first one.

Malcolm Thawne is the identical twin brother of Barry Allen who had been separated from birth. After the doctor failed to save Charlene Thawne's baby, as he was too intoxicated to do so, Wanting to be fair, the doctor told the Allens that the Barry's twin was stillborn and gave the twin to the Thawne's who named him Malcolm. Malcolm had been raised by con-artists who sold a blue colored jelly which stemmed from a blue flame that they could create. Finally, when he was seventeen, during one of their many arguments in which Malcolm asked to be taught the power of the blue fire, his mother told him that he had been adopted. Going to Fallville, knowing that was where he was born, Malcolm sought out Dr. Gilmore. The doctor told him of his origins, and the enraged lad killed the doctor for having picked him to give to the other family. After travelling the world and getting into trouble with the law along the way, Malcolm finally decided to locate and confront Barry. Envious about Barry's life, Malcolm got a job as a janitor and watched him from afar and grew to resent him for having the life that he should have had. After a confrontation with his adoptive grandmother, she teaches him the true extent of the blue flame's power and is able to use it to absorb/steel speed from other speedsters. 

Obviously, the whole blue flame concept is a bit too strange for them to do. But it could go well with the element of Barry being his own worst enemy. This could also be an illusion to the Allen/Thawn rivalry we had seen in the comics. 

Edward Clariss is Jay Garrick's "Reverse-Flash". He is actually referred as the Rival though. Edward Clariss was a university professor who had supposedly recreated the chemistry formula that granted Jay his speed. Bitterly rejected of his claims by the scientific community, Clariss became a criminal, wearing a darker version of the Flash's outfit. His version of the formula, however, proved to be temporary, and he was defeated when it was expended. 

Jay had stated that he had been fighting against Zoom for about two years. It could ultimately be simple as facing a big threat that neither one of them (Jay or Barry) have no idea to face, at the moment. Or he could be combined with the well-known diabolical Jonathan Allen/Chambers a.k.a Johnny Quick who was also the father of Jesse Quick.

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