Hell Hound is a Medium fiend with lawful evil alignment and Challenge Rating 3 in the 2014 SRD. This stat block includes AC 15, 45 hit points, speeds, traits, and actions.
Bite - Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) piercing damage plus 7 (2d6) fire damage.
Fire Breath - The hound exhales fire in a 15-foot cone. Each creature in that area must make a DC 12 Dexterity saving throw, taking 21 (6d6) fire damage on a failed save, or half as much damage on a successful one.