[dancing spawn of ungoliant]

I haven't voted yet so let's see if I can organize my thoughts while I write and come into some proper conclusion. On the first time I read LotR Gothmog was an orc to me. I didn't really think if it was logical because I like to read the books as a story not an encyclopedia. Besides, I think Gothmog sounds very orcish. At least it doesn't sound like a name of a Ringwraith. However, after doing a little brainwork I don't believe that he was an orc.

Do you know what was a Lieutenant's place in historical battles? Was he at the front marching with the troops or was it possible that he came to the battlefield some other way (here: flying a Fell beast)? I don't know how much we can trust Frodo's observations of just one Nazgul. He fell asleep or passed out while Morgul's host was crossing a bridge. Before it Frodo seemed to be so caught in the fear of failing that I don't know if he had noticed if there had been two Nazguls instead of one.

Originally Posted by RotK, The Siege of Gondor

It was no brigand or orc-chieftain that ordered the assault upon Lord of Mordor's greatest foe.

Does this mean that usually there could have been just some extra ugly and nasty orc or man leading a charge against an enemy?

But later on almost the same sentence is repeated.

Originally Posted by Rotk, The Battle of Pelennor Fields

But it was no orc-chieftain or brigand that led the assault upon Gondor.

It is clear that this is a very important battle. Losing isn't an option. I doubt that Sauron would have trusted an orc or ordinary man to be Witch King's substitute. Of course Sauron didn't think that anything would happen to WK but still.

Originally Posted by The Battle of Pelennor Fields

Gothmog the lieutenant of Morgul had flung them into the fray; Easterlings with axes, and Variags of Khand, Southrons in scarlet and out of Far Harad black men like half-trolls

Gothmog leads men. I don't think men would take orders from an orc. It seems natural that a man, a Black Numenorean if you like, would command them. Also, I think that Tolkien would have mentioned if Gothmog was a Nazgul. Like SoN said:

Never was Gothmog described in the way of the other Nazgul (striking fear into the heart of his enemies, etc.)

I'm going to vote for Black Numenorean.