Lungorthin (whose name only appears once) was a Balrog in an early stage of The Silmarillion. He is not mentioned in the later Silmarillion, or the published Simarillion.
After Húrin was captured by Morgoth, Lungorthin smote him on the mouth. Lungorthin bore the title Lord of Balrogs.
J.R.R. Tolkien, Christopher Tolkien (ed.), The Lays of Beleriand, "I. The Lay of the Children of Húrin",
pp. 98, 102-3