The Tale of the Water Kelpie's Wife.
A water kelpie is a dangerous, shape-shifting spirit thought to inhabit the rivers and streams of Scotland. Taking on the shape of a beautiful horse, it lures anyone incautious enough to try to ride it to their death. One in particular is said to have been banished from the River Ness by St. Columba, and became associated with the loch. This kelpie is the river horse of our story.
The water kelpie's wife hated living in the dank, dark, dismal depths of Loch Ness.. In an attempt to devise a plan to ease her plight, her husband went to the surface and took the form of a beautiful black stallion with a long, flowing mane and tail. A stone mason saw the stallion and, unable to resist, leapt onto its back, and found himself stuck fast. With a crack like thunder, the horse dove to the bottom of the loch, taking the stone mason with him. There, the water kelpie struck a deal with the terrified man. The stone mason agreed to build a fireplace and chimney that reached to the surface of the loch in exchange for fish for the rest of his life. To this day a patch of water on Loch Ness never freezes- it's where the chimney comes out.