The Kelpies, also called "Water Kelpies", are a type of water spirits described in the folklore of both Scotland and Ireland, and they are said to haunt rivers and lochs and were usually malevolent - although rare cases of benevolent or neutral Kelpies exist. Although dangerous the Kelpies was not regarded as evil as the Each Uisge creatures, which was a similar type of water-demons in Scottish mythology.
A Kelpie often took the form of a lost pony with either black or white pelt, in each form it was said to have a mane that was constantly wet - if required a Kelpie could take the form of an attractive woman or man to lure people to their fate and the spirit specialized in drowning its victims before devouring them, leaving only a heart or liver behind.
While in the form of a pony, a Kelpie was said to tempt people to ride it but as soon as their victim touched the creature they would become stuck as if glued onto the beast, the Kelpie would then go into a charge towards the nearest body of water, leaping into it and drowning its unwilling rider before feasting.
Like many of Scotland's legendary demons, the Kelpie is regarded as a type of fairy as well as a spirit of the dead, presumably that of people or animals that have drowned - they were greatly feared by those who believed in them and even in fairly recent times some people were afraid of approaching unfamiliar ponies in the wild.