Yeth Hound
Yeth Hounds are evil and dangerous spirits that roam the Spiritworld and the Shadow World, and can ocasionally be found in the World of Mortals in places where the influence of the Shadow World is strong. The appearance of a yeth hound is that of a large black dog the size of a pony, but with an almost frail looking and suprisingly light body. While very few people ever get a good close up look at their faces, their heads are of a much smoother shape than dogs or wolves and more resembles a hyena, though some have even described their faces as almost human-like. The ears of yeth hounds are very long and pointed, making them appear like horns when viwed in front of a lit background.

Yeth hounds are always seen or heard only during night and never stay outside close to dusk or dawn. With their ability to magically fly, they can be almost completely silent and their dark color makes them very hard to spot except under a full moon. They usually hunt in groups of three to nine, silently gliding through the shadows until they pick up the trail of potential prey. Their most dangerous power is their barking and howling, which though sounding similar to dogs can cause great terror even in the strongest and most hardened warriors. Unless they are particularly hungry and desperate for food, yeth hounds use this ability to send their prey into panic and single out one or two targets which they will chase until they collapses from exhaustion. Unless slain or driven off, only the approach of dawn can make a pack of yeth hounds abandon their hunt.

While they seem to be perfectly at home in the Shadow World, yeth hounds appear to be actually native to the Spiritworld. While they eat any kind of meat, yeth hounds highly favor to hunt both spirits and mortal humanoids, with shie and pixies being especially highly prized by them. As much as the flesh of humanoid creatures, they enjoy the hunt and the chase and prefer to let their prey run and allow it some moments of respite, only to stir it up again with their howling until it can not run from them any longer. Sometimes they appear to lose track of time and have to abandon their hunts as dawn approaches, though some people have voiced the suspicion that they do it deliberately to spread tales that it is possible to outrun them if one can manage to stay ahead of them for long enough.

Being almost invisible and completely silent when they chose to, almost nothing is known about the behavior of yeth hounds when they are not attacking their victims. It's even hard to estimate the size of their groups when hunting, but about three to nine individuals seems to be the most likely number. There are some stories of particularly infamous and cruel shie lords who keep yeth hounds in their castles in the Shadow World, but it is unclear if they are trained guard and hunting dogs, or merely share homes for mutual benefit.

These are one of those creatures that have always been one of "those critters in the 3rd Edition monster manual that never showed up anywhere and most people forgot exist". But with clearly putting them in the spiritworld and putting other spirits on their list of prey, I think they can become quite interesting and maybe even iconic critters.
And fighting with them would be totally like that first ringwraith encounter in the Lord of the Rings movies.