Here are 2 URLs showing zebras that have been trained to ride and drive
DRAUGHT AND RIDING ZEBRAS
The Myth of the Wild Zebra | If Wishes Were Horses
For some English visiting or working in Africa in the late 1800's and early 1900's, zebras were trained because the horses brought to Africa died of diseases caught there. Many physicians used Zebras to pull their buggies when they worked in the bush. In Montreal's history museum, now in the archives, there is a photo of 2 officials riding zebras together in Africa because the horses brought there did not survive.
For other English in Africa, Zebras were trained just for the novelty of training them.