ceffyl y dwfr
"Water-horse." In Welsh folklore, a malevolent water-horse which inhabits streams and lakes. It would induce the unwary traveler to mount him, and then
soaring into the clouds, or dashing over river and mountain, would suddenly melt into air or mist; and precipitate his rider into destruction.
- Motley, J. (1848). Tales of the Cymry: With Notes Illustrative and Explanatory. London: Longmans, p. 150