In Welsh folklore, what water horse lures riders to a watery grave?

In Welsh folklore, the water horse that lures riders to a watery grave is the Ceffyl Dŵr.

The Ceffyl Dŵr is a supernatural horse associated with mountain pools, waterfalls, rivers and sometimes the seashore. Folklore often describes it as grey, black or glowing, with a white mane. It entices a traveller to mount, then races away, flies into the air, vanishes into mist or plunges toward deadly water.

Its behaviour varies by region. Some Welsh traditions portray it as vicious, while others make it a troublesome or even helpful spirit. Stories also say it could change shape, appearing as a goat or frog, and that anyone who captured it for work would eventually be dragged to death.

The name is sometimes written ceffyl dwfr or ceffyl-dŵr. It is commonly compared with Scotland’s kelpie and Ireland’s each-uisge, but those are related water-horse traditions rather than the specific Welsh answer.