Monsterween: Cú Sidhe
Did you know that today's Halloween traditions draw roughly on the old Gaelic holiday of Samhain? It's a truly ancient holiday-- structures dating back to 3,400 BC in Ireland are aligned to the sunrise on November 1st. During this celebration, livestock were brought in from pasture and butchered for the winter, souls of the dead were thought to revisit their homes, and the fairies were able to enter the mortal world and do all kinds of nonsense, including In honor of the fair folk (and in the hopes that they will not abduct our children) we've got some fey hounds to set loose on your poor 5E villages, or to "collect" some deserving adventurers for tea.
Their baying is heard for miles around. In town, children and new mothers are hurried inside, followed quickly by the rest. Horses break free from the livery and run for the hills, even as carriages on the road ratchet up to breakneck speeds. Birds and other woodland creatures cease their usual activities and come to watch. Then it appears, a wolf the size of a bull dancing nimbly out of the briar, sniffing the air for its quarry and licking its muzzle delightedly.
Fey Beasts. A cú sidhe is an enormous lupine creature with a shaggy coat of black, white, green or blue fur, though some have blood-red ears, muzzles or paws. Often they will be draped in a harness of briars and thorny vines. Despite their monstrous size and strength, they also possess a fey speed and equipoise, as well as a startlingly intelligent bearing.
Hunting Hounds. The fair folk keep cú sidhe in the same way that noble humans might keep hunting dogs. But the fair folk don't hunt foxes or rabbits. Instead, the cú sidhe are generally meant to bring home infant babies, new mothers (to be used as wet nurses for fairy children), unlucky mortals who catch their owners' interests, lost souls, or even renegade fairies.
Retrievers. Unless heavily wounded or frightened, a cú sidhe rarely attempts to kill its quarry-- they are trained to bring prey home still breathing and can be startlingly gentle with their jaws. Cú sidhe generally love to be cut loose on a hunt, though whether that joy is a sadistic glee or merely a deadly playfulness is usually a reflection of their owner.