A fire demon is a small evil creature that serves Satan. They appear as a cross between a lizard and amphibian comprised of flames. They are able to travel through fire.
When Count Dracula stole Dolores away from Satan, the priest of the Fellowship of the Black Mass commanded Satan to appear. Disgusted that a mortal would command him, Satan summoned a fire demon. The demon escaped the fire and attacked the priest for a short amount of time then disappeared back into the flames. Most likely, it was summoned merely to scare the mortal.