: I think it likely that he was some kind of cunning, intelligent Olog-hai. Mainly because I don't think anything else would have been dumb/tough enough to take on such a name. And trolls were the toughest that Sauron had for his armies.

If it was a troll or human it had to be higher in rank than 5 of the nazgul. The witch king kept 5 (i think) of the nazgul in Minas Morgul. Khamul and another one was at Dol Guldur and one stayed with Sauron as his messager. So the second in command of Minas Morgul would probably be a ringwraith because it is unlikely that any man or troll was to give orders to a ringwraith.

Maglor