While on this wiki, I have seen many examples of people (I am unsure who) naming Glorfindel as the possible slayer of Lungorthin; this is very likely incorrect for numerous reasons.
When Tolkien first wrote The Fall of Gondolin, he conceived of a large number of Balrogs, as Christopher Tolkien states in his commentary following the Fall of Gondolin (I leave it unitalicized for the reader's ease); 'The early conception of the Balrogs makes them less terrible, and certainly more destructible, than they afterwards became; they existed in "hundreds"...and were slain by Tuor and the Gondothlim in large numbers: thus five fell before Tuor...three before Ecthelion's sword, and two score slain by the warriors of the king's house.'
The claim that the slain Balrog might be Lungorthin is based upon a note that states that there were only three (I was unable to find the note, despite looking in the Index under Balrog for both Unfinished Tales and The Letters); where as in this state there were at bare minimum forty-eight. Thus, it is very unlikely that the Balrog that Glorfindel slew was Lungorthin; while still possible, an equally viable claim could be made that Durin's Bane was Lungorthin.
Another reason is that the draft for The Fall of Gondolin that was shown within The History of Middle-earth was very old; the copy was dated as being made in 1916-1917, and revised in 1920 when Tolkien was only twenty-eight! (fun fact, older than Australia), whereas the note was most likely made much later, which would have given both the Legendarium and Tolkien himself copious time to mature, develop, and change by the time the note was written.
