Yeth hounds are hunters that shun the sun but revel in the fear they cause hunting by night. It’s not said what plane they’re from, but you can be sure they lurk throughout the Ethereal and Lower planes (they certainly understand Infernal and Abyssal well enough). Though they don't play well with other canines, they do work as allies for nearly every other evil creature. The result is a really flexible monster you can slot into almost any role, from vampire sidekick to unseelie hunter to cult ally to Ethereal Plane menace.
Also, there is a yeth hound of folklore, though it deviates from the RPG version. (For one thing, RPG yeth hounds have heads.) But since yeth hounds are outsiders, there’s no reason they couldn’t be the spirits of unbaptized children. Likewise, a singular yeth hound (especially an Advanced specimen) might be the local black dog of your campaign hometown, and confronting it might be one of the party’s last acts close to home before they move on to adventures in the wider world.
Barghests seem to have a party surrounded, when suddenly a pack of yeth hounds drives them off. The yeth hounds don’t leave the party alone, however. Instead the pack circles back to test the heroes. If they show no fear and appear cruel in nature, the hounds propose an alliance. Otherwise the hounds attack, doing their best to hide their power of flight in the hopes that the adventurers will make themselves more vulnerable by fleeing up a tree or a precarious hillside.
A wounded psychopomp manifests to a party of adventurers, asking for help. Yeth hounds have been savaging area children and collecting their bones inside an ancient windmill. Those children too young to have been baptized return as immature hounds (use the Young template) themselves, because no nosoi can get close enough escort the souls to their proper afterlife. If the party does end up confronting the hounds, they will do their best to force the confrontation to happen out on the rotting sails of the mill. Further investigation reveals the windmill has some connection to the night hags of legend.
Evil entities known to favor yeth hound allies include the vampire Mikros, who flies with his hunting pack astride a zombie wyvern; a half-fey elven summoner who claims to be an erlking; Comrade Yevin, a socialist spectre who seeks souls to join his spawn collective; and the Gardener, an intelligent scarecrow witch.
I know I’ve been a bit slow on the comments and reader mail lately, but that’s because I’m trying to make sure I stay on track as he hurtle toward Z. Speaking of which, I’m in Florida for the next four days. (The alarm to make my flight goes off at 4:00 AM, and it’s already 10:15. Yikes.) My plan is to get the Yeti and Yithian entries up on schedule, but bear with me if I’m a hair tardy on either of them.
You know what else I love? “Basic” D&D’s spectral hounds, which in practice weren’t too dissimilar from yeth hounds. When I get back I’ll try to look up which Dragon Magazine issue featured an amazing illustration of one attacking the Princess Ark.