Ceffyl Dwr is a water horse in Welsh folklore, similar to the Kelpie in Scotland.
The book Folk-lore and Folk-tales of Wales by Marie Trevelyan states that the Ceffyl Dr can shape shift and even fly, though this varies depending on where you are in Wales. For example in North Wales he is represented as being rather formidable with fiery eyes and a dark forbidding presence whilst in South Wales it appears he is seen in a more positive light, at worst a cheeky pest to travelers and at best, as Trevelyan puts it, 'luminous, fascinating and sometimes a winged steed.
The Ceffyl Dwr is said to inhabit mountain pools and waterfalls. Even though it appears solid, it is seen to evaporate into the mist. In one form of the legend the Ceffyl Dwr, as a horse, leaps out of the water to trample and kill lone travelers.
Another form of the legend reports that the Ceffyl Dwr entices the unwary traveller to ride him. Flying into the air, the Ceffyl Dwr evaporates, dropping the unfortunate rider to his death