This submission of the mythology sketchgroup is the Gytrash: 

A kind of sinister fairy from the folklore of northern britain. At night, it can take the form of a large horse, donkey or sometimes a shaggy black dog with webbed feet. It has huge saucer-shaped eyes and walks with a splashing sound. It is generally described as lurking silently by the side of the road waiting for unwary travellers; however, some stories speak of it in a more positive light as helping to lead lost travellers to safety. There is a memorable description of the Gytrash in Charlotte Bronte's novel Jane Eyre. (see also Padfoot)

I chose the horse iteration of the creature, playing with the idea of a drowned animal seeking revenge upon all others to suffer a similar fate. The saucer-shaped eyes seemed like a chance to play with the eerie alertness of an unnaturally emaciated wretch.