Balrogs: Valar, Maiar or Spirits?

Question

I encountered the following bit, regarding Balrogs, in The Letters of J.R.R. Tolkien: Letter 144:

The Balrogs, of whom the whips were the chief weapons, were primeval spirits of destroying fire, chief servants of the primeval Dark Power of the First Age.

which was quite surprising given that it always seemed that they were fallen Maiar. Indeed, at the end of Valaquenta it says:

For of the Maiar many were drawn to his splendour in the days of his greatness, and remained in that allegiance down into his darkness; and others he corrupted afterwards to his service with lies and treacherous gifts. Dreadful among these spirits were the Valaraukar, the scourges of fire than in Middle-earth were called the Balrogs, demons of terror.

So, while not all corrupted Maiar became Balrogs (e.g. Sauron), all Balrogs were initially Maiar. Although, it is also surprising that there are called Valaraukar - apparently from Vala and rauco (monster), which is a bit odd.

A possible explanation would be that, in Letter 144, Tolkien was referring to the Ainur as primeval spirits - not to be confused with the primeval spirits such as Ungoliant; or the origins of the Balrogs were not yet determined.

So my question is: is it just a matter of wording, was there an evolution of the origins of Balrogs or something else?

Answer 1

It's a matter of wording. Every Balrog is a Maia.

The Maiar are all Ainur, who are also described thusly in Valaquenta:

The Great among these spirits the Elves name the Valar, the Powers of Arda

So here we have the most powerful of the Ainur to descend into Eä - the Valar - being described as "spirits."

And later, the Maiar are explicitly referred to as spirits:

With the Valar came other spirits whose being also began before the World, of the same order as the Valar but of less degree. These are the Maiar

And a little bit later the Balrogs are included in the descriptions of some of the Maiar who were corrupted by Morgoth:

For of the Maiar many were drawn to his splendour in the days of his greatness, and remained in that allegiance down into his darkness; and others he corrupted afterwards to his service with lies and treacherous gifts. Dreadful among these spirits were the Valaraukar, the scourges of fire that in Middle-earth were called the Balrogs, demons of terror.

However, the word "spirit" is also used to describe non-Ainur. The Elves have spirits that burn (Feänor's birth drains his mother's spirit so much she actually passes away).

So Balrogs are spirits; they are also Maiar, and they are also Ainur. They are not Valar.

Answer 2

The Balrogs are most definitely Maiar, but at the same spirits. But not Valar.

Tolkien described the Balrogs as Úmaiar, where "ú" is the negative prefix for words in Quenya.

They now become 'evil spirits (Umaiar) that followed him' - but he could 'multiply' them. The term Umaiar, not met before, stands to Maiar

[...]

... the conception that the Balrogs (Valaraukar) were powerful spirits from before the World; the Balrogs are described as the chief of 'the evil spirits that followed [Melkor], the Umaiar'.”
_{Morgoth's Ring}

The difference between the Valar and the Maiar can be seen here in full, but in short, the great were called Valar and the lesser Maiar.

The Great among these spirits the Elves name the Valar

[...]

... of the same order as the Valar but of less degree. These are the Maiar, the people of the Valar, and their servants and helpers. _{The Silmarillion II Valaquenta}

Finally, spirits in Tolkien's Legendarium are quite complicated. There were a few types of spirits. Again, and in-depth explanation can be found here, but in case here is a summary. The most common two being the Fëar and Ëalar, which are the spirits that required a body (hroä, found within the Children of Ilúvatar) and the spirits which did not require a body (the Ainur).

The spirits mentioned in your quotes would've been the Ëalar, which Morgoth corrupted to form primarily the Balrogs, although spirits that don't become Balrogs are mentioned.

These were the (ealar) spirits who first adhered to him in the days of his splendour, and became most like him in his corruption: their hearts were of fire, but they were cloaked in darkness, and terror went before them; they had whips of flame. Balrogs they were named by the Noldor in later days.
_{Morgoth's Ring: Of the Coming of the Elves}

Followed by the footnote for the word ealar:

'spirit' (not incarnate, which was fea, S[indarin] fae). eala 'being'.
_ibid

Other spirits that are mentioned which are unclear what they are are those that Yavanna summons into the Olvar and Kelvar, similarly the spirits Sauron traps into the werewolves. Furthermore, Tolkien, in Morgoth's Ring, discusses that the Eagles and Huan, if not maiar, were beasts that had been taught language by the Valar.

Finally, there are the Nameless Things, and Bombadil, which are mentioned and never explained.