The original mythological yeth hounds were headless dogs, said to be the spirits of unbaptized children. They would run around in the woods, making wailing noises. They were featured in both Dictionary of Phrase and Fable and in The Denham Tracts.
This art is from Monster Manual II.
Medium fiend, neutral evil
Armor Class 15 (natural armor)
Hit Points 45 (7d8+14)
Speed 40 ft., fly 60 ft.
Str Dex Con Int Wis Cha
16 (+3) 15 (+2) 15 (+2) 7 (-2) 14 (+2) 10 (+0)
Skills Perception +6, Stealth +6
Damage Resistances bludgeoning, piercing and slashing from nonmagical weapons that aren’t silvered
Senses darkvision 60 ft., passive Perception 16
Languages understands Infernal but can’t speak
Bay. When the yeth hound howls or barks, all creatures except other fiends within a 90 ft. radius must make a DC 12 Wisdom save or become frightened for 2d4 rounds.
Keen Hearing and Smell. The yeth hound has advantage on Wisdom (Perception) checks that rely on hearing or smell.
Magic Weapons. The yeth hound’s weapon attacks are magical.
Pack Tactics. The yeth hound has advantage on an attack roll against a creature if at least one of the hound’s allies is within 5 feet of the creature and the ally isn’t incapacitated.
Bite. Melee weapon attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8+3) piercing damage. If the target is a creature, it must succeed on a DC 13 Strength saving throw or be knocked prone.
A yeth hound is a sinister flying canine that inhabits the lower planes. These large dogs are black in color, with faces that strangely bear characteristics of a humanoid’s face. Their eyes glow red, and their pointed ears curve to resemble horns. Their ghastly baying can be heard for a mile or more.
Packs of the beasts somehow find their way to the material plane, where they inhabit remote wilderness.
Fearsome Pack Hunters
Yeth hounds form packs, with the strongest hound as pack leader. They truly only need to feed once per lunar cycle, but are eager carnivores. Their favored prey are humans, demihumans and fey, but will hunt any warm-blooded creature they encounter. They willingly run with evil creatures such as night hags or vampires.
They hunt much like other pack hunters do, though their ability to fly allows them to cover a larger area than any normal beast. They will occasionally drag dead prey back to their lair to be consumed at their leisure.
No natural predator hunts a yeth hound, and many unnatural predators will avoid them as well.
Packs of yeth hounds lair underground, away from the hateful light of the sun. Yeth hounds fear the sun, and nothing can persuade one to venture out during the day. If a hound is exposed to sunlight, it fades away to the ethereal plane, unable to escape it on its own. As a result, packs only hunt at night, and always return to their lair before dawn.
Yeth hounds don’t value treasure, so any valuables found in their lair will be random discarded remnants of their prey.