In her 1973 book Folk-lore and Folk-tales of Wales, Marie Trevelyan says that the Ceffyl Dŵr was believed to shapeshift and even fly, although this varies depending on region. For example, in North Wales he is represented as being rather formidable with fiery eyes and a dark foreboding presence, whereas in South Wales he is seen more positively as, at worst a cheeky pest to travellers and, at best, as Trevelyan puts it, "luminous, fascinating and sometimes a winged steed".
The Ceffyl Dŵr is said to inhabit mountain pools and waterfalls. Even though it appears solid, it can evaporate into the mist. In one form of the legend the Ceffyl Dŵr, as a horse, leaps out of the water to trample and kill lone travellers.
Another form of the legend reports that the Ceffyl Dŵr entices the unwary traveller to ride him. Flying into the air, the Ceffyl Dŵr evaporates, dropping the unfortunate rider to his death.
- Evans, Thomas Christopher (1887). History of Llangynwyd parish. Printed at the "Llanelly and County Guardian" Office. p. 170.
- Rose, Carol (2001). Giants, Monsters & Dragons, An Encyclopedia of Folklore, Legend and Myth. W. W. Norton & Company. ISBN 978-0-393-32211-8.
- Trevelyan, Marie (1973). Folk-lore and folk-stories of Wales. EP Publishing. ISBN 9780854099382.