You know I was thinking there might be a chance we might have already seen a modified version of a Hrothgar character model. The devs have been known to use character models of the player races, including ones in development, and modify them slightly for boss fights. Case in point, the first example of a Viera character model in-game is not Fran but Tsukuyomi:
Her Primal form is nothing more than a Viera character model scaled up in size with some other slight modifications. This is not the first instance of this, for example Iceheart uses a Elzen character model that has been modified. Now let's take a look at Byakko's Human Form:
Scale it down to size of a Roedyragen or an Au Ra, give it two normal arms and you end up with something that actually matches what little description we have of the Hrothgar and has the same foot structure as the Viera.