Transforming into an equine guise is not a new idea - not by any means! Here are some accounts of people transmuted into horses and similar forms dating from the dawn of Greek myth to the Nineteenth Century.

The Tale of Cronus and Philyra, and the First Centaur Ocyrrhoe, the Centaur's Daughter
The Mother-in-Law who became an Ass The History of Sidi Nu'uman
Saint Macarius of Egypt Tales from Siberia
The Wicked Wife of Vamadatta The Tavern-Keeper of Schwarzenstein
Donkey Cabbage The Witch of Troms Church
The Woman who became a Horse The Last Execution in Dornoch
Margaret Grant of Scotland The Witch Trials of Blocula
Prince Badr Basim and Queen Lab