We actually have very hard confirmation that it's not Haurchefant who Zenos is possessing. During one of the interviews Yoshida had at E3, he talked about how he does like giving players true hints about where the story is going next, because that anticipation is fun for the players. He said he doesn't like giving players false hints if he and the story team can help it, because that creates a false anticipation. His example of a "true hint" was the zoom-in on a certain mask at the end of 4.3. He specifically told the cut-scene creators to do that shot so that it was a very obvious hint to the players about who the Shadowhunter really is so long as they recognized the mask. His example of a "false hint" was how he read some players were theorizing that Zenos had taken over Haurchefant and wished that he and the story team could have come up with a better way of showing that whoever Zenos was possessing wasn't Haurchefant so that players wouldn't be mislead.

As far as where the person Zenos is possessing could have come from... we see Ishgardean Elezen in the Ala Mhigo dungeon. It's probably one of them.