Balrogs

A Balrog is a powerful demonic creature found in the Lord Of The Rigs series, although the film only portrayed one Balrog they are actually a demonic species in their own right - although they are noted as being a race that have undergone heavy changes by Tolkien as he began to alter them from a race that was once numerous enough to be described as "hosts" to beings that "at most" numbered seven at a time.

Another aspect of the Balrog that changed through time was their size and physical nature, with later versions becoming more elemental in nature - fiery and near indestructible (this was the type chosen for the movie, it would appear).