In Tolkien's The Fellowship of the Ring, the type of demon that
Gandalf battled in the Mines of Moria. As morgandorf notes
below, the balrogs were Maiar corrupted by Melkor/Morgoth. The
passage cited seems to indicate that not all Melkor's Maiar
flunkies became balrogs, only those whose nature was imbued with
fire (whether by Melkor or Eru is not clear). The word "balrog"
is Sindarin, rooted in Quenya "valarauko" or "malarauko" ("demon
of power"). Compare Latin "val-", as in "valence" and "valor".
In fantasy RPG, drawing heavily on Tolkien for many of the
creatures, I have heard of balrog interpretations for at least 5
systems: Dungeons and Dragons, RuneQuest, MERP, Palladium
Fantasy, and a Middle-Earth scenario for Sid Meier's
Civilization II. In D&D "balrog" is usually synonymous with
"Type VI demon".  See also Balrog Award.  Balrogs were in fact
fallen Maiar:   --The Silmarillion  My interpretation of others
in the paragraph above is "other Maiar".  Whether or not they
had wings is still a hotly debated t