This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Ceffyl Dwr (Welsh)
Ceffyl Dwr is a water horse in Welsh folklore. It is most commonly considered a counterpart to the Scottish kelpie, but has also been compared to the Irish puca, the Irish and Scottish each uisge, and the Manx glaistyn. The ceffyl dwr is said to inhabit mostly mountain pools and waterfalls, though it has been known to frequent rivers and seashores as well.
It was believed to be a spirit assuming the shape of a horse, usually grey or black with a white mane, sometimes glowing or winged, to entice unwary travellers to ride him. Once mounted it would either carry its rider great distances very quickly, or fly into the sky, then evaporate into air or mist, dropping the unfortunate rider to their death. The ceffyl dwr could be caught and put to work, but would always escape eventually and drag its captor to their death.