I once had a thread theorizing that Flemeth might be an Old God; it made perfect sense in the context of DA:O (and in the fact that seemingly no one else can learn to shapeshift into a dragon, canonically speaking).
I do not really believe she could be Dumat, but perhaps she is another Old God who managed to avoid being sealed away, or escaped. Or, she might be, as others suggested, Fen'Harel. As powerful as she is, the blight concerned her in DA:O, though, really, faced with an overwhelming swarm of darkspawn, who wouldn't be concerned?
Of interest is that she seemingly doesn't leave the Wilds often, if at all. She does have that legend behind her, and she has her own story about it. Her story suggests she's an abomination (of vengeance, maybe), but it might indeed be that she's more than that. So we can guess that she might be somehow tied to the Korcari Wilds, and it could even be that she didn't go to a demon for help, but to an Old God or even an elven god, or some other entity we know nothing about.
For all we know, Flemeth is the reason the Black City became corrupted and Dumat felt the need to send in mages to investigate, only to have the corruption then travel to him and also corrupt him.
It might be interesting, though, if the entity that was bound to Flemeth was actually some form of god or old god, which would mean that no, she's not human and also not an abomination. But I am like others; I think we'd know if Dumat hadn't been really slain. I guess it's possible, if someone had guessed or known about the Dark Ritual. But if they did know about it, then we have to ask how they knew. Did someone tell them? Or did someone who had something to do with the origin of the darkspawn in the first place know enough to say so? In either case, how did Flemeth find out? Or is she just well-learned enough by this point that she can crank out all kinds of theories on magic and its application that no one else could, due to their lack of time and experience that Flemeth has in spades? That's equally likely.
Personally, I think that one quest with the underground magical city in DA:2 gives some suspicion that the corruption and the darkspawn began with something those apparently magic-using dwarves did (if they were dwarves). Where and how Flemeth ties into this, I don't know. But while I suspect she does have at least minor ties with the Old Gods, if she isn't one herself, I still can't quite bring myself to agree with the theory that she is Dumat. If she is an Old God, she's one of the undiscovered, and unslain ones, and clearly she intends to remain that way. And yet, if she is? Then as long as she lives, there will be darkspawn trying to rise up and corrupt her. She'd fit quite well as the Old God of Mystery, Razikale, more so than as Dumat, the Old God of Silence. Or as Urthemiel, the Old God of Beauty, but we know she can't be him, since he was the archdemon we defeated in DA:O. Of course, we know of seven Old Gods that the Tevinters worshiped. There may have been others that were not known about, or simply not worshiped. Many larger pantheons have some minor deities who aren't given as much importance.