Lungorthin is mentioned in HOME 3, The Lays of Beleriand: and he's called the Lord of the Balrogs, but we also know Gothmog was blessed with this title as well. Chris Tolkien speculates that Lungorthin may have just been a Balrog lord. Is there any evidence that may support that Lungorthin may have been the first Balrog lord?
Lungorthin or 'pale blue flames of horror' (1) is only mentioned once by JRR. It came from a revision when thousands of Balrog's existed and in various shapes and forms. Lungorthin for instance was wreathed in blue or white flame and was master of the guard at Melkor's throne, staying deep underground at Thangorodrim.
It's likely that Lungorthin was either revised out of the lore altogether or as the potency and scarcity of Balrogs increased in later revisions his role was merged with that of Gothmog.
Either way, Gothmog and his role in the Simarillion makes him the first and greatest of the Balrog lords.