Kelpies are malevolent shape-shifting creatures that usually appear as horses (often black). When an unsuspecting person sees and mounts a kelpie, the creature will plunge into a body of water, drowning the victim before eating them. Kelpies are also referred to as water horses and they always bear some token of the lake or sea such as water dripping from their coats or seaweed in their manes.
They are not to be confused with the Loch Ness Monster as depicted in, "The Water Horse, Legend of the Deep." This film calls the Loch Ness Monster beast a "water-horse" which works in the film since the animal has a horse like-gait while skimming over the lake surface. However, he is simply not a kelpie and the film seems to blend kelpies and Loch Ness Monsters by allowing them to share a name and by the tale the handy-man relates about a "water horse" which sounds like a story about a kelpie.
Perhaps the Loch Ness Monster is one form the shape-shifting kelpie may take, thus explaining why no one can find her?! But this theory holds little water, for if the monster were truly a kelpie, why did he only appear as the Loch Ness Monster? And why did he not drown any one? Therefore, the creature from the film is not a kelpie and caution should be used with the title "water horse." The ideas set forth in the film just do not fit with lore, instead they form their own theories about the Loch Ness Monster by borrowing from kelpie lore.
The film is well-made with a nice story. My only caveat is, don't treat it as an authority on water horses! If you would like to watch it, here is a link to information on the film: