tolkiens legendarium - Balrogs: Valar, Maiar or Spirits?

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

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.