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Howlers totally feel like a 3e monster, probably because of the art. I suppose they might have shown up in AD&D too, it’s not like the game was ever shy about adding one more dog monster to the pile.
Howlers are spiky, sapient quadrupedal predators that feed on fear. They’ve evolved to hunt in a way that’s as terrifying as possible. Younger howlers are quick to kill because they enjoy the spike of fear the victim feels when it dies, but older specimens are sophisticated enough to hold whole villages hostage with the threat of death-by-howler and feed off the ambient terror.
The most legendary howler of all is known as Terror Incarnate. This creature managed to devour the fear felt by an entire army as it died, and this excess power turned it into something like a demigod of terror. Its eventual progeny inherited its powers.
There’s some fuzzy edges in their lore. It mentions howlers attacking people in the middle world and in the Underdark, mentions associations with shadar-kai, but gives the creature an Elemental origin. I think they might fit better as either Natural or Shadow. Fortunately origin doesn’t influence stats. Alternatively you might go the other way and make them a type of demon.
As mentioned above, Howlers are Elemental Magical Beasts. They have Darkvision and a Speed of 8. They understand Abyssal but can’t speak.
Howler Dread Hound
One of those young specimens that kills too quickly. They gather in huge packs to massacre whole villages at once. They are Medium Level 9 Minion Controllers with a speed of 8. At Int 5, they’re sapient but dim.
As with all “dog monsters”, their basic attack is a bite. This one does a bit of damage and slides the target 1 square on a hit. They also have a Piercing Spines trait that deals the same damage as the bite to anyone who misses them with a melee attack. The ability that gives them their name is the Dread Howl encounter power, a fear attack that catches every enemy in a Close Blast 5 and targets Will. A hit does a bit of psychic damage and makes the target grant combat advantage for a turn.
These are minions, so there will always be a lot of them when they appear. A bit of orchestration could ensure PCs grant combat advantage for most of the fight.
Howler Doom Mastiff
An older, fully developed howler who has learned the value of prolonged psychological torture. They roam ruins and tunnels in small packs, looking for victims to terrify. They are Large Level 13 Skirmishers with 129 HP and a speed of 8.
Doom Mastiffs are large enough to ride, and they have the Mount keyword. As they have Int 9, I think this is more of an equal partnership thing than the rider “keeping” the howler as a mount or pet. The Guarding Spines trait gives the mastiff’s rider a +2 bonus to AC. Its Piercing Spines does 10 damage to anyone who misses the howler or its rider with a melee attack.
Their basic Bites are a little weak but can be used as part of a Loping Bite attack that allows the mastiff to shift half its speed and bite. Their Terrifying Howl is a lot stronger than the dread hound’s: it deals 10 ongoing psychic damage to those it hits, and makes them grant combat advantage (save ends). Targets adjacent to the mastiff can’t save agains this. Finally, this is a (recharge 6+) power instead of an encounter power. So it might happen more than once!
Being all about that delicious fear, the mastiff will open with a howl and then will do its best to stick to one of the affected PCs, using move actions and Loping Bites to do so. It’s smart enough to prefer a squishy victim over a defender, too.
Howler Terror Incarnate
The one that devoured an army’s collective fear and became a demigod of terror. It’s a Large Level 22 Lurker with 155 HP.
The terror’s Shadow Spines work a lot like that of its lesser relatives, but deal necrotic damage. It also has a Terror Incarnate aura (3) that deals 10 psychic damage per turn to any enemies inside.
Their basic bite deals the same damage as that of the level 13 doom mastiff, which I’m almost sure is a typo. It can also possess people with the Shadow of Terror power. This is a melee attack that can only be used against people who can’t see the howler, and who haven’t been possessed during the fight yet. On a hit, the howler is removed from play and the target is dominated (save ends). The target’s at-will attacks deal an extra 1d8 psychic damage and target Will instead of their original defenses. The posession ends when the target passes a save or takes radiant damage - when this happens, the howler reappears adjacen to them. The target is then dazed (save ends).
Their final attack is the Death Howl, another blast attack. This one does immediate psychic damage and dazes (save ends) on a hit.
Shadow Lope (recharge 4+) provides the ability to set up for a Shadow of Terror attack: it allows the howler to become invisible for a turn and move its speed.
Howlers are far from the first dog monsters to show up in D&D. They’re not even the first dog monsters with fear powers. Still, you can use them in your campaign in place of wargs and other similarly-leveled canine menaces for added variety. The Doom Mastiff’s ability to act as a mount also makes it an possible replacement for a Nightmare. Anyone who would ride an Evil Fire Horse would also certainly ride an Evil Fear Dog.