Must a fairy always resemble a human?
Certainly not! Nor must a fairy take on human characteristics. We are speaking of immortals here, and immortals are not bound to think, exist, or behave as mortals do. So why not assume the aspect of an animal? And why not an enormous Black Dog?
Old Shuck, Black Shuck, Old Shock, Snarleyow, the Dog Fiend . . . whatever his name, this fellow is a very famous fairy-beast of Old English lore. Throughout history, across the British Isles, stories have been told a malevolent black dog with flaming eyes (or, sometimes, green eyes) that stalks the coastline or countryside, often lingering near graveyards, dark forests, or sights of murder.
Most often, he is an omen of doom. Sometimes, however, he has served as a protector. Either way, he is a popular Anglican figure, so popular, in fact, and so often cited throughout the centuries, that he is listed as a cryptid: a creature whose existence has been suggested but is unrecognized by scientific consensus . . . and, truth be told, often regarded as highly unlikely.
Unlikely or otherwise, Old Shuck has a vital life in the world of lore and literature, and is this life any less vital than the reality in which we exist?
See this chilling account from back in 1577:
This black dog, or the divel in such a linenesse (God hee knoweth al who worketh all,) running all along down the body of the church with great swiftnesse, and incredible haste, among the people, in a visible fourm and shape, passed between two persons, as they were kneeling uppon their knees, and occupied in prayer as it seemed, wrung the necks of them bothe at one instant clene backward, in somuch that even at a mome[n]t where they kneeled, they stra[n]gely dyed.
A Straunge and Terrible Wunder by the Reverend Abraham Fleming
Possibly the most famous reimagining of this terrible beast was Sir Arthur Conan Doyle's brilliant, Hound of the Baskervilles. The titular hound, oppressor of the Baskerville family, haunting presence on the wild moors, is the very likeness of Old Shuck . . . at least, as far as meets the eye!
Sherlock Holmes will come upon a different answer, given time.
I was pleased to recently rediscover this dreadful fairy in the children's book, The Beast of Noor, written most chillingly by Janet Lee Carey. Ms. Carey takes the myth in her own, interesting direction, but the core of the legend, the sense of strange, otherworldly reality, remains intact.
My own world features a pair of Black Dogs, sibling monsters that haunt the Paths of the Netherworld save when sent on various errands by the Death-in-Life himself. Once they are sent after their prey, the never stop until they run him down!
And have I mentioned that I live with my very own daily omen of doom?
Oh no! The Black Dog is pursuing!