How about we do as Tolkien hints we might, and use the term "Boldog" to refer to a very minor Śmaiar, one that is below Sauron, below Gothmog and the other Balrogs, and certainly WAY below their mutual master, Melkor-Morgoth? In other words, let's use it as generic term for the fourth tier of Ainur-gone-bad.
The First Tier is Melkor-Morgoth, the Alpha and the Omega of Evil. Peerless in all Arda, and that's taking the good guys into account.
Then we have the Second Rank, the powerful Maiar gone bad. Sauron, Ungoliant, and maybe one or two others that we don't know about fit in here.
Then we have the Third Rank, the Balrogs. We could also put them in the Second Rank, as junior partners. Gothmog (the Balrog), after all, seems close in importance to Sauron in command of the First Age's armies.
Now we come to a less easily proven level, what I shall call 4th Tier Śmaiar. These are the ancient spirits, Ainur who came into Arda, and who formed the bulk of the "many" Maiar who were seduced to evil by Melkor, but who don't seem to have been powerful enough to individually become a menace or threat. Thuringwethil and Draugluin might well fit into this category. So also would these Ork-Captains that Tolkien writes about, these powerful, strong, extra-large-and-nasty Orks that formed the original Ork hosts and commanded and interbred with the later ones. Let us use Boldog as a generic term for any of these Ork-captains, just as Balrog may be used for any Śmaiar of the Third Rank.
Being so low on the scale of power, these Boldogs would likely have been tied to a single incarnate form (hence their ability to reproduce and mate with other proto-Orks), and would thus have definitely been "slay-able", just as the more powerful Balrogs were. Also, as with the more powerful Balrogs, these Boldogs would not have been like to reincarnate themselves.
Most therefore, would not have survived the First Age. Those who did would likely have become the great Ork-lords of the Second. What with Sauron's invasion of Eregion, attacks from Westernesse, the War of the Last Alliance, and their own infighting, it would appear doubtful that many would have survived into the Third Age.
Perhaps Gothmog II was one of the last (or THE last) of these petty-Śmaiar, these minor Ainur-gone-bad. Still quite fearsome compared with men, but certainly within a man's power to kill, and certainly able to be dominated by Sauron, and even the Witch-king.
On the same topic, is it possible that Azog of Moria was another surviving Boldog? Hence his "kingly" status. Also, his son Bolg, as at least a half-Śmaiar, would then have had enough of a headstart on the competition in what was surely a mean and deadly fight for the leadership of the Orks.
Neither Azog nor Gothmog II need have been "the" Boldog, the one who fought Beleg and the wardens on the Marchs of Doriath, but surely it is possible that one or both them were "a" Boldog, or at least Orks with a stronger strain of Boldog in their blood. A kind of Orkish Line of Elros, so to speak.
*By the way, nice to see a fellow Lego/Tolkien fan.
