Source: Sword & Wizardry Tome of Horror Complete (Originally from the AD&D 1e Fiend Folio).
Also appears in Fiend Folio for 3e and Pathfinder Bestiary 4.
Packs of two headed dogs spreading rotting death throughout the amber deserts. These nocturnal hunters stalk barren wastelands looking for prey. First the pack attacks. Bites then turn into infections. The pack retreats and waits. The victim slowly dying as its flesh begins to rot. After their prey is dead the Death Dogs feast on their corpses.
Combat: The Death Dogs are Challenge Rating of 3 in Sword & Wizardry. They have two bite attacks that can infect it's opponent with rotting death. This disease is quite potent. If you fail a save you lose 1d6 points of constitution each day with -5 saving throw each subsequent save. In Sword & Wizardy with low attributes a couple of failed saving throws is almost sure death.
You can however be healed with powerful curative magic or finally make a saving throw. With complete bed rest you regain one point of constitution each week.
Inspiration: Roving bands of vicious dogs can create a certain amount of dread. If the group is traveling through a barren terrain you can have the pack perform hit and run tactics. In the middle of the night a half dozen of these dogs attack. After a round or two they retreat. Maybe one or two PCs come down with rotting death. Each night you hear the howls of the Death Dogs as they wait patiently for it's prey to fall from illness.
You can also have the antidote be a mixtures of the Death Dog blood and some herbs. So now the group must track down the pack to slay one for the cure. Another possibility is the group stumble upon an entrench caravan. Half of the caravan is suffering from Rotting Death. They are in no position to continue unless a cure is found and the pack of Death Dogs are dealt with.