Why not Ecthelion?

[Galin]

So just, because he refers to Glorfindel's battle with 'demon' hardly implies he planned to change it from a Balrog to some other beast.

And incidentally, to be fair to the other person who raised the possibility, Tolkien actually appears to have crossed out the word Balrog in one instance [in the late text concerned], as well as employ demon throughout. I can't really easily explain why he should feel the need to do that...

... although again I still don't buy it myself [even if Balrog numbers were drastically reduced]...

but let's not [including me, if I have] simplify this person's suggestion of a possibility

And that's all it was, if I recall correctly.

[gondowe]

I think the matter is easier (my humble opinion). If we would want (as it stands in the section of the New Silmarillion) to create an coherent text, the number of Balrogs must be treated as a mixed version. I mean, the 3 or 7 note is that, "a note" , not developed by the professor, if the text had been rewritten we had the "truth", but we haven't, only know that the number is wanted to be reduced in accordance with the new "strength" of the demons opposite to the Lost Tales version. I think with no more data rewritten, we can assume 7 equal to 10 or 12 for example.
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