This is what's written in Arthur Spiderwick's Field Guide to the Fantastical World Around You about Kelpies.
Kelpie (Family: Equidae)Edit
A ghastly water spirit assuming the form of a grayish black horse, the kelpie drowns then kills anyone who attempts to ride it.
It is often spotted wandering along the shores of rivers or lakes, appearing to be a lost pony. Kelpies can be identified by a constantly dripping mane and skin, which is like a seal's but cold to the touch.
The kelpie can be heard wailing before a storm and is able to cause water to rise high enough to flood.
If you manage to bridle a kelpie, it can be forced to do your bidding, but woe betide you should it slip the harness.
- Lake Kelpie (Equus carnivorus)
Like the Unicorn, Kelpie have cloven toes instead of hooves.
A Kelpie is about the size of a ponys. The front foot is 13 cm long, while the back foot is 15 cm long. The stride is approximately 55 cm long, somewhat smaller than the horse's.
Known Kelpies Edit
When two children, a 10-year old girl named Edith and a 12-year old boy named Peter, appeared missing, the polices' search revealed a large number of Human remains in the water of a lake somewhere near a town located northeast of the Spiderwick Mansion. Locals reported the lake was a popular swimming hole where many people, even horses, would take a cooling bath in the summer. It was clearly evidence of a Kelpie.
When Arthur heard of this, he became interested in such a manner his wife would describe it as "grisly and macabre", according to a note from Arthur, written in March the 1st, 1929. The next day, he wrote maybe his wife was right, and that he was worried about how he would present his researches to science and be taken seriously. He also said at the time he was no longer sure what was real and what was illusion.
Behind the scenesEdit
In Celtic mythology, the Kelpie was a water spirit able to take the form of a water horse in order to lure people into riding on its back. The 'water horse' would then drown its victums in the nearest lake or stream. Sometimes the horse transformed into a beautiful young woman and lured men. A very similar creature is the Scandinavian Näck, appearing in the form of a naked handsome young man sitting in the creek playing a violin. Sometimes it turned into a Bäckahäst ("Brook Horse") to drown human in a similar manner. In Germany are the Nix or Nixe appearing as a shape-shifting water creature. The Nix in some tales liked to drown people in their lakes, but in others were friendly and harmless.