An Evil of the Ancient World Is Come Again

From the Valaquenta, an excerpt from "Of the Enemies" -

Yet so great was the power of [Melkor's] uprising that in ages forgotten he contended with Manwe and all the Valar, and through long years in Arda held dominion in most of the lands of the Earth. But he was not alone. 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 Valarankar, the scourges of fire that in Middle Earth were called the Balrogs, demons of terror.

Recent scholarship among Prof. Tolkien's papers has unearthed more information concerning this evil:

Of the Balrogs' part in the tale of woes in Middle Earth, much is lost. However, one name has survived the erosive effect of time on the memory of its Peoples and is held even now by Elves, Dwarves and Men in dread and horror: Bambi. Reckoned amongst the foulest of Morgoth's servants, he took the form of a gentle, dewy-eyed, four-legged creature of the wild. Thus, all the more cruelly for his outwardly adorable guise, Bambi passed hither and thither across the lands of Middle Earth, laying waste the gardens of Arda and replacing them with a black desert in which no life could survive. Despite the fall of his master into the outer darkness, this fiend still stalks the lands with hooves of hell, seeking to devour what it may.

Thus it cometh again, the Bambi Balrog of Morgoth.