Yep, very many Balrogs appear in
The Book of Lost Tales, and this is one of those ideas that persisted for decades, still appearing in certain of the 'latest' writing in some instances.
For example:
'There came wolves and serpents, and there came Balrogs one thousand,...' (Of the Fourth Battle: Nírnaith Arnediad). This is but one example that, as far as I know, was never revised by JRRT for whatever reason, altered to →
'There came wolves and wolfriders, and there came Balrogs, and dragons...' (Of The Fifth Battle) 1977 Silmarillion.
And then there is Tolkien's own revision to
Annals of Aman of (possibly) 1958, and his marginal note, as we know, to confuse things nice and good.
