(From 5th Edition Monster Manual - 2014):

Fire-breathing fiends that take the form of powerful dogs, hell hounds commonly serve evil creatures that use them as guard animals and companions.

Monstrous, fire-breathing fiends that take the form of powerful dogs, hell hounds are found on the battlefields of Acheron and throughout the Lower Planes. On the Material Plane, hell hounds are most commonly seen in service to devils, fire giants, and other evil creatures that use them as guard animals and companions.

Burning Hunger. Hell hounds hunt in packs, feeding on any creature that appears edible. They avoid potentially dangerous foes in favor of targeting the weakest prey with their savage bite and fiery breath, demonstrating a relentless determination as they pursue that prey to the bitter end.

When hell hounds feed, the flesh they consume stokes the infernal fires that burn within them. When a hell hound dies, that fire consumes the creature’s remains in a billowing eruption of smoke and blazing embers, leaving nothing behind but scorched tufts of black fur.

Evil to the Core. Hell hounds are smarter than mundane beasts, and their lawful nature makes them good at following orders. However, a hell hound’s evil disposition means that the creature can’t be trained to be anything other than a ruthless killer. If a hell hound isn’t allowed to indulge its malevolent hunger, it quickly abandons or turns against its master.

(From Monster Manual v3.5 - 2003):

The creature resembles a big, powerfully built dog with short, rust-red fur; its markings, teeth, and tongue are sooty black. It has red, glowing eyes. 

Hell hounds are aggressive, fire-breathing canines from the plane of Acheron. Specimens are frequently brought to the Material Plane to serve evil beings, and many have established indigenous breeding populations. A typical hell hound stands 4-1/2 feet high at the shoulder and weighs 120 pounds. Hell hounds do not speak but understand Infernal. 

Combat: Hell hounds are efficient hunters. A favorite pack tactic is to surround prey quietly, then attack with one or two hounds, driving it toward the rest with their fiery breath. If the prey doesn’t run, the pack closes in. Hell hounds track fleeing prey relentlessly. 

A hell hound’s natural weapons, as well as any weapons it wields, are treated as evilaligned and lawful-aligned for the purpose of overcoming damage reduction. 

Breath Weapon (Su): 10-foot cone, once every 2d4 rounds, damage 2d6 fire, Reflex DC 13 half. The save DC is Constitutionbased. 

Fiery Bite (Su): A hell hound deals an extra 1d6 points of fire damage every time it bites an opponent, as if its bite were a flaming weapon. 

Skills: Hell hounds have a +5 racial bonus on Hide and Move Silently checks. *They also receive a +8 racial bonus on Survival checks when tracking by scent, due to their keen sense of smell. 

NESSIAN WARHOUND:

The Lords of the Nine, the great devil princes who rule over the Nine Hells, keep vast kennels of hell hounds. From the fiery pits beneath the palace of Asmodeus in Nessus, the lowest of the Nine Hells, come the Nessian warhounds, a terrifying breed of hell hound. Nessian warhounds are coal-black mastiffs the size of draft horses, often fitted with shirts of infernal chainmail. Nessian warhounds resemble hell hounds, except as otherwise noted. 

Breath Weapon (Su): 10-foot cone, once every 2d4 rounds, damage 3d6 fire, Reflex DC 21 half. The save DC is Constitution-based. 

Fiery Bite (Su): A Nessian warhound deals an extra 1d8 points of fire d a m a g e every time it bites an opponent, as if its bite were a flaming weapon.

(From Monstrous Manual - 1993):

Hell hounds are fire-breathing canines from another plane of existence brought here in the service of evil beings.

A hell hound resembles a large hound with rust-red or red-brown fur and red, glowing eyes. The markings, teeth, and tongue are soot black. It stands two to three feet high at the shoulder, and has a distinct odor of smoke and sulfur. The baying sounds it makes have an eerie, hollow tone that send a shiver through any who hear them.

Combat: Hell hounds are clever hunters that operate in packs. They do not bay like normal dogs while hunting. They move with great stealth, imposing a -5 penalty to opponents’ surprise rolls. One or two of the pack sneak up on a quarry while the others form a ring around it. The first hell hound then springs from ambush, attacks the nearest victim, and attempts to drive the others toward the rest of the pack. If the prey does not run away, the rest of the pack closes in within 1d4+2 rounds. If hell hounds are pursuing fleeing prey, they might bay.

Hell hounds attack first by breathing fire at an opponent up to 10 yards away. The fire causes 1 point of damage for each of the hell hound�s Hit Dice. A successful saving throw vs. breath weapon cuts the damage in half. The hell hound then attacks with its teeth. The hell hound can continue to exhale flame while biting. If the hell hound rolls a natural 20 on its attack roll, it grabs a victim in its jaws and breathes fire on the victim.

Hell hounds have a variety of defenses. They are immune to fire. Their keen hearing means they are surprised only on a 1 or 2 on 1d10. They can also see hidden or invisible creatures 50% of the time.

Habitat/Society: Hell hounds are native to those extradimensional planes notable for their hot, fiery landscapes. There they roam in packs of 2d20 beasts. The hell hounds on the Prime Material plane are summoned there to serve the needs of evil creatures. Most of them later escape to the wild.

Hell hounds may have 4 to 7 (1d4+3) Hit Dice. The more Hit Dice a hell hound has, the larger it is and the more damage it causes. Each pack is led by a 7-Hit Die hell hound. The leader drives off other 7 HD rivals, who form their own packs.

The diet of hell hounds is similar to that of normal canines. They roam a wide area of 1d10+4 square miles centered on their den. Pack territories may overlap.

They do not easily reproduce on the Prime Material plane. Only 5% of encounters include puppies. Such puppies are born in litters of 2d4. They burp flame uncontrollably at least once a day. The flames are harmless aside from the tendency to set fire to anything flammable in the area. Newborn puppies are at 10% of the adult growth; they quickly grow an additional 5% each month and reach full adult growth (4 HD) in 1� years. While growing they can attack with their incendiary bite. Hell hound puppies up to two months old inflict 1 point of damage. Older ones add an additional 1 point for each additional six months of growth.

Prey is usually eaten where it is slain, though hell hounds occasionally haul a carcass back to their den for later meals. Hell hounds are also similar to normal canines in that they may act as retrievers. Some objects are specifically sought; this is especially the case in trained hell hounds. Other hell hounds are simply playful and use the retrieved items as toys. They especially like noisy bags and pouches filled with their late victims’ treasures. Flammable containers eventually burn and spill their contents in or around the den. Parchments are rarely found here unless protected by nonflammable containers.

Ecology: Hell hounds have little place in the ecology of the normal world. They are dangerous annoyances prone to cause fires wherever they hunt. Hell hounds cause more forest fires than any other creature except for humanoids. Hell hounds have their uses, though. Because of their ability to easily detect hidden or invisible creatures, hell hounds make excellent watch dogs, especially for intelligent monsters such as fire giants.

Hell hounds can be domesticated if raised from puppies, but there is a 10% chance each year that domesticated hell hounds go wild.