Balrog

The Balrog was one of several of his kind and originally a spirit of the Maiar who was seduced by the Dark Powers, thus transforming into a horrible creature out of fire and darkness. The Balrog featured in the Valinorian Chronicles is the Balrog of Moria, which was also known as Durin's Bane. It was said to look like a gigantic human with leather-like wings, hooves instead of feet and goat-like horns on its head. It wielded a sword and a whip, both made out of fire.

Prior to the first book (Freedom) the Balrog is believed to have been slain by Gandalf, who stood on the Bridge of Khazad-Dûm and fought this creature in order to make sure that the Fellowship made it safely out of Moria.

Spoilers

 * The Balrog/Durin's Bane will return in later books, battling both Gandalf and Freedom on different occasions
 * More Balrogs might be featured later on (this has not been decided as of yet)