[Keeper of Dol Guldur]

If Aragorn was wise and learned in the tales of old, why did he not recognize that it was a Balrog? Maybe because every last one was thought to have been destroyed in the War of Wrath? How many of them participated in the War of Wrath? Zero. That happened two ages prior. There are fouler things in the depths, probably things worse than the Watcher at Westgate, that they anticipated it could have been. After all, so many ageless demonic creatures had been brought up and nurtured by the likes of Morgoth or Sauron in the past, like the Nazgul hell-hawks, the wolf Carcharoth, and even older, more nasty creatures like Shelob still roamed the lands. Well, she didn't roam. But you see the point. There were various other things it could have been, besides a Balrog. But as an obvious thing made of dark shadow and racing with flame, that could blow Mithrandir's magic and bust down Balin's chamber, had to A. Be large, and B. A fire spirit. There weren't too many other things that fit the description they saw for themselves in Moria. Also, the Balrog was demonic looking and very much mighty, and what does Balrog (more specifically Valaraukar) mean? Exactly.