In the early story recorded in Book of Lost
Tales 2, "Fall of Gondolin", there were many Balrogs and of
less power than in
the later stories. They numbered in the hundreds and were slain by Tuor
the elves in large numbers: five fell to Tuor's axe, Dramborleg, three
Ecthelion's sword, two score went down to the warriors of the king's
These are called "demons of power", feel fear and pain, have iron
have whips of flame (this trait continues), and claws of steel.
In later stories, their concept was changed, even as the machine dragon idea changed to organic, with the dragons of the Fall of Gondolin in the Silmarillion said to be "of the brood of Glaurung". In a quote from BoLT 2, Fall of Gondolin, "The idea that Morgoth disposed of a 'host' of Balrogs endured long, but in a late note my father said that only very few ever existed - 'at most seven'."
References: BoLT 2, "Fall of Gondolin", Silmarillion