In Coxwell's collection of Siberian and Other Folk-tales, there are no fewer than six equine transformations.
What Happened to the Khan and the Poor Man's Son is a Kalmuck tale in which a mother and daughter who rob the khan and his friend are punished by being transformed into asses and forced to perform hard labour for several years before their penance is deemed adequate.
In the Finnish tale The Magic Bird, three young sorceresses attempt to murder a man who keeps reviving. His revenge is to turn them into three black mares and have them harnessed to heavy loads until he is satisfied.
The Merchant's Sons is a Finnish story of two brothers, one of whom tries to win the hand of the tsar's wicked daughter. The girl does not like her suitor and endeavors to have him killed, but he turns her into a beautiful mare which he and his brother ride. In the end he turns her back into a girl and marries her.
In the Mordvin story Enchanter and Enchantress a magician catches his wife with her lover: she transforms him first into a yellow dog and then into a sparrow. When she finally releases him, he retaliates by transforming her into a chestnut mare which he obliges to haul stones and logs for the building of their new house. At the end of the story they both regain their human forms and settle down peacefully together. Could be an innovative new way to solve marriage disputes!
In the tale Tsopan, Tsopan's wife turns him first into a duck and then into a dog for not bringing home any food. The dog catches a devil who gives him a felt whip with powers of transformation. He releases himself and goes home to find that his wife has married a giant. He transforms both into asses and allows the whole village to use them for hauling firewood. Ultimately he forgives them and restores their original forms.
Finally, in the Estonian tale The Peasant and the Fiend, a strange gentleman sells a peasant a fine horse and a whip with the instruction to beat the animal and not to feed it. The horse works well for three years until one Christmas the peasant takes pity on it and gives it a piece of bread, whereupon it becomes a man who explains that as punishment for being a wicked landlord the Devil had turned him into a horse. The Devil then appears, turns the man back into a horse, and they both vanish. Landlords beware!