Other Evidence William Isn't Human
When the full-length trailer for Westworld season 2 was released, it included a brief shot of the Man in Black holding a gun to his head. It was hard to guess what events might lead to this eventuality, but it certainly fits with the theory of the Man in Black being a human-host hybrid. After all, every time Delos learned what he was, the knowledge made him break down, so this scene could take place after the Man in Black discovers what he has become - especially in light of what William said about how no one should live forever. It also echoes his recent interaction with the new version of El Lazo, which ended with El Lazo choosing to shoot himself in the head rather than accompany the Man in Black in the search for further truth.
Another possible clue is a brief moment in the season 1 finale, when Bernard and the Man in Black passed one another at the party celebrating the launch of "Journey Into Night." The Man in Black glances back at Bernard for a moment, and a second later Bernard looks back at the Man in Black. It's a curious interaction since, as far as the audience has seen, these two have never met. It would make sense, however, if they were actually intimately connected - say, by Bernard being in possession of the control unit for the Man in Black - but had both had their memories of one another wiped.
On a characterization level, it would be very interesting for the Man in Black to discover that he's actually a kind of host himself. His character arc so far has taken him from being the kind of young man who believed in the world of Westworld so sincerely that he actually fell in love with Dolores, to a man so disillusioned that he can slaughter child hosts without compunction and call Dolores a "thing" to her face. He's also the character who is most intimately connected with the technology used to transfer human consciousness into hosts, and one of a very small number of characters who is even aware of the technology.
One of the major reasons why the Man in Black was believed to be a host in season 1 was the fact that he's based on a character from Michael Crichton's original 1973 movie, the Gunslinger, who was played by Yul Brynner. This character was far simpler than William - effectively just a monster who stalked the movie's human protagonist through Westworld and its neighboring worlds before finally being defeated. However, the fact that the original story's Man in Black was a robot certainly lends weight to the theory that Ed Harris' character is a robot as well - albeit one with a human consciousness.
There are other characters who could potentially be the human-host hybrid, of course. Perhaps it's actually William's daughter, Grace, who demonstrated a keen awareness of just how easily a host could be mistaken for a human in her first appearance on the show. Perhaps it's Delos executive Charlotte Hale, about whom we still know relatively little. Or maybe Robert Ford actually made a host copy of himself before he was killed by Dolores, and that copy is hiding somewhere in the park.
Do you have your own theory about who the human-host hybrid is? Let us know in the comments!
Westworld airs Sundays at 9 pm on HBO.