Maybe that's a normal enemy and not a Boss at all. I mean, we don't know how big this thing is, it could be very small, smaller than Gabriel, it could be just as big as a troll or it could be Gargantuan and be another Titan like Boss. Maybe it is a normal enemy, mabe it is a Titan fight, maybe it is Legion, maybe it is Zobek's final form or maybe just another boss, who knows? Or maybe it won't appear in the game at all.