"The Water Horse"
More a subject of cryptozoology (the coolest thing since aliens) than of folklore, the Ceffyl-Dwr (as it's commonly known in Wales) is a horse gifted with flight but, alas, without wings. Usually spotted near a waterfall or pool of some sort this crippled pegasi is a bit of prankster, Loki style, and will occasionally allow mortals to "catch" him, ride him but inevitably will toss them to the ground...usually resulting in more of an ego bruising than bone-breaking.