This is a sinister fairly that from the folklore of Northern Britain. At the night Gytrash can take on the form of a large house, donkey or sometimes a black shaggy with webbed feet. It has huge saucer-shaped eyes and walks with a splashing sound. It described as lurking silently by the side of the road waiting for unwary travellers. They are some stories that speak of it in a positive light as helping to lead the traveller to safety. There is a memorable description of the Gytrash in Charlotte novel Jane Eyre (see also Padfoot).
John and Caitlin Matthews, The Element Encyclopaedia of Magical Creatures, 2013, Gytrash, p 220