The Witch's Bridle transforms not only people, but individual bones into serviceable horses. Often these are bones of horses--shoulder-bone, jawbone, legbone--but occasionally they are human bones. The famous churchman Sira Halfdan of Fell once bridled the hip-bone of a man, turning it into "a willing horse that could go as well over the sea as on land." It is also said that the Witch's Bridle is the only way to fully tame a nykur or nennir, the magical white horses that come out of the sea.
To make a Witch's Bridle, one story says, cut three narrow strips of skin off the spine of a newly dead corpse and twist each one while pulling it through a hole in a skull--usually the ear hole. (The witch would use an already prepared skull, rather than that of the fresh corpse; of course she'd have one at hand.) Braid the strips into reins. Next, flay off the dead man's scalp and fashion it into the head-piece of the bridle, with the hair left on. The bones at the root of the tongue (the hyoid bones) are used for the bit, while the hip bones make the cheek pieces--the Witch's Bridle follows the form of the classic Icelandic bridle, with its large cheek pieces.
When properly pieced together, the bridle can be fastened onto "any man or beast, stock or stone, and it will go quicker than lightning wherever one wants to go." In practice, however, the bridle must have been programmed by magic words to go to one particular destination, for in no instance in the tales does the witch-ridden bone, beast, or man deviate from course--even when the rider is not the witch, as in the tale above, in which the farmhand turns the tables on his mistress and bridles her for the ride back home from Elfland. Perhaps the spell was recited while the skin for the reins was being pulled through the ear hole, thus allowing the skin to "hear" the instructions.
For more shakes and shivers (and a few love spells), read last year's Halloween post about the Icelandic Witchcraft Museum: http://www.nancymariebrown.blogspot.com/2013/10/icelandic-witchcraft.html