(From Mordenkainen's Tome of Foes - 2018):

A far-off wail precedes the sight of a howler. Even at a distance, one’s mind cringes at the sound and fills with horror at the realization that the noise is drawing closer. When howlers go on the prowl, courage isn’t enough to stand up against them, and even one’s sanity is at risk.

Prowlers from Pandemonium. These nightmare creatures, native to Pandemonium, can also be found on most of the Lower Planes, because of the many fiends that capture them and train them as war hounds. Howlers can be domesticated, after a fashion, but they respond only to brutal training during which they are forced to recognize the trainer as the pack’s undisputed leader. A trained pack then follows its leader without hesitation. Howler packs course over the battlefields of the Blood War and also serve evil mortals who have the power and the savagery to command their loyalty.

Brutal Hunters. Howlers rely on speed, numbers, and their mind-numbing howl to corner prey before they tear it apart. The howl floods the minds of its victims, making complex thought impossible. They can do little more than stare in horror and stumble around the battlefield in a search for safety. Any task more demanding than that is beyond their capability. Fiends especially prize howlers for this reason, because for a few crucial moments in a battle, their howls can neutralize an enemy’s ability to use spells and other powers.

(From 3.5e Monster Manual - 2003):

This creature looks like some gaunt, bestial hound or feline, with a mane of bristling quills. 

Howlers live on planes where chaos and evil hold sway; they originate on the plane of Pandemonium. These beasts hunt in packs, racing through caverns to wear down their prey and rend it to bits. 

A howler is about 8 feet long and weighs about 2,000 pounds. 

Although they are surprisingly intelligent, howlers do not speak—they only howl. If there is a language within the howls, as some have suggested, even spells cannot decipher it. Howlers understand Abyssal. 

Combat: Howlers attack in groups, for they are cowardly and cruel. They prefer to charge into combat, race out, and then charge in again. A howler’s natural weapons, as well as any weapons it wields, are treated as chaotic-aligned and evil-aligned for the purpose of overcoming damage reduction. 

Quills (Ex): A howler’s neck bristles with long quills. While biting, the creature thrashes about, striking with 1d4 of them. An opponent hit by a howler’s quill attack must succeed on a DC 16 Reflex save or have the quill break off in his or her flesh. Lodged quills impose a –1 penalty on attacks, saves, and checks per quill. The save DC is Dexterity-based. 

A quill can be removed safely with a DC 20 Heal check; otherwise, removing a quill deals an extra 1d6 points of damage. 

Howl (Ex): All beings other than outsiders that hear the creature’s howling for an hour or longer are subject to its effect, though it does not help the howler in combat. Anyone within hearing range of a howler for a full hour must succeed on a DC 12 Will save or take 1 point of Wisdom damage. The save DC is Charisma-based. The save must be repeated for each hour of exposure. This is a sonic mindaffecting effect. 

Training a Howler:

Small and Medium infernal creatures such as quasits, abyssal orcs, and even succubi sometimes use howlers as mounts or pack animals. Larger and more powerful demons use them like hunting dogs. 

Although intelligent, a howler requires training before it can bear a rider in combat. To be trained, a howler must have a friendly attitude toward the trainer (this can be achieved through a successful Diplomacy check). Training a friendly howler requires six weeks of work and a DC 25 Handle Animal check. Riding a howler requires an exotic saddle. A howler can fight while carrying a rider, but the rider cannot also attack unless he or she succeeds on a Ride check. 

Carrying Capacity: A light load for a howler is up to 460 pounds; a medium load, 461–920 pounds; and a heavy load, 921–1,380 pounds. A howler can drag 6,900 pounds.

(Planescape: Planes of Chaos Monstrous Supplement - 1994):

Howlers are the keening brood-hounds or Pandemonium, the singers of dementia who cluster around the great rocky spires in those plunging caves. In that eternally night-lit plane the howlers are drawn to shriek their madness to the moonless heavens.

The howlers are gigantic beasts, four-legged, with the burly backs of oxen and scales glinting through their matted red fur. This fur spreads into a thick ruff of trembling quills that frame their simian faces. Their front claws are the clotted knots of coarse fingers too long crnshed under the beastly weight, their back feet are hooved. When they cry, they rock back until they almost stand and throw their muzzled faces toward the missing sky. Lips pursed into a perfect O, they howl the voices of madness.

Combat: Although brutish and cruel, howlers aren’t particularly aggressive beasts. Scavengers by nature, they seldom attack groups larger than their own numbers or any that seem more capable of defending themselves. When a howler pack does attack, it usually singles out a weak victim, while trying to hold the rest at bay. If this proves impossible, the howler pack breaks and flees.

This doesn’t mean, however, that howlers are weak opponents. Far from it — their own cowardice makes them formidable opponents. When a howler pack does attack, it is determined and ruthless. All energy is devoted to bringing down the prey.

In actual battle, howlers can choose from several attack forms. Normally they rush in on the first round of combat, attempting to get within biting distance as soon as possible. The rush and the round that follows are the two most dangerous moments or any howler attack. As they rush forward, their neck bristles rise to form a spined shield, reducing their Armor Class to 3. If the howlers move 120 reel or more in the charge, they gain a +2 bonus to their chance to hit and their damage is doubled for carrying the charge through. Finally, while in their frenzied charge, the howlers are immune to all morale checks.

Once in combat, howlers fight by snapping their powerful jaws and savagely slashing with their erect neck quills. The quills can only be used on those at the creatures’ forward flanks and aren’t particularly accurate (-2 on the chance to hit). Each successful hit jabs the person with 1d4 quills, and each quill causes 1d4 points of damage. Creatures struck by the quills must make a saving throw versus breath weapon for each quill. For each save that is failed, the quill remains embedded In the victim. The victim’s attack rolls are reduced by 1. Removing quills takes 1d6 rounds and causes an additional 1d3 points of damage.

Habitat/Society: Howlers are beasts native to the tunnels of Pandemonium and are not naturally found on any other plane. Even within Pandemonium they aren’t found everywhere. They stay far from the settled regions and the small tunnels, preferring the larger curved passages that pass for plains and grassland. On these plains they hunt in packs; for they are carnivorous scavengers.

There is no hard and fast size for a howler pack, as is the way of the animal kingdom. Packs range from 2 to as many as 20 members. This number includes bulls, females, and kits. (Of a pack, it is the bulls who defend the others, hence the small number met in a normal encounter) The packs scour the land. searching for anything edible. As news of their migrations travels through the tunnels, other dwellers in Pandemonium brace themselves for the agony that is sure to come.

Howlers are aptly named, for their howls cut through the whistling wind with the pitch of madness. Their cries have the same effects as the winds of Pandemonium, gradually driving those who hear them to insanity. Whenever a howler lets loose its cry, all within hearing distance, whether indoor or out, must make a check as if they were exposed to Pandemonium’s winds. A howler’s cry lacks the full strength of the winds — their keening pitch can never push a chancier beyond Stage 3 (Hysteria). Characters who have already reached Resignation exhibit all their tics and twitches when the howlers sing. Even on other planes, a howler’s effect travels with it.

It is fortunate that howlers don’t bay all the time. In that regard they are more like common dogs. They howl and keen occasionally, when the circumstances are right. As pack animals, they howl when they are lonely — kept in the stable apart from their masters, for example. They howl during the rutting season and when their territory is challenged. They howl when cornered. The DM has final say over when the beasts bay to the blackness.

Ecology: Howlers would be just another bane of Pandemonium were it not for those in the tunnels who can capture and tame the beasts to be good riding and pack mounts. A howler has the carrying capacity of a draft horse, and is far better adapted to traveling the twisted tunnels than a simple horse.

Some wizards claim the howlers sing secrets of the planes, a code concealed in their pitch and their keening. Others think the wizards have been howled mad by their studies.