Word on the street (that street being Wikipedia Lane) has howlers arising from the Planescape’s Planes of Chaos before graduating to the 3.0 Monster Manual and thence to the Bestiary 2. As low-CR outsiders, they’re another nice introduction to the evils of the Abyss without being full-fledged demons. They also pair well with any monster immune to their mind-affecting cursed howls.
The young vampire Lord Kendrick of Ness was a huntsman in life, with a brace of hounds. In undeath he keeps a pack of howlers instead. As the howlers feed on fear rather than flesh, they don’t deprive him of potential kills and soften up the psyches of his own meals quite nicely.
A cult of demonologists have set loose a pack of howlers to harry the followers of Lucius, the Sovereign of the Nine. The City Watch needs help—they may despise the Lucians, but the faith does have the crown’s charter, so the Watch is obligated to protect them. And if they don’t stop the attacks soon, the Lucians have threatened to stop moving through official channels and summon barghests in retaliation, which would lead to bloody chaos in the streets.
The realm of Infernus is a Hell on earth, literally—in this reality, the Mount of Heaven and the Pit of Hell are not other planes but merely far distant but very real nations. (In fact, one often sees the translucent, ectoplasmic souls of the dead on the road toward one or the other, making their last pilgrimages.) While ancient prohibitions keep most devils and angels in their respective realms, the nations that border Infernus are plagued with howlers every autumn.
—Pathfinder Bestiary 2 159
Sadly not having owned any Planescape gear (such a mistake!), I can’t recall which 2e plane howlers were natives of. I’m guessing Pandemonium, which was definitely one of the creepier of the Lower Planes. (Any place stuffed with tunnels, madmen, gnolls, derros, whispered secrets, and ever-shrieking winds is okay in my book.)
On the other hand, if their evil were toned down I could easily see howlers being perfect stat blocks for boar-q-pines in an Avatar: The Last Airbender RPG.