There's a radio / podcast show I listen to called 'horses in the morning' which is one of 8 or so shows they have on the 'horse radio network'. One for driving, general, barn, dressage, eventing, equestrian legends, etc
The host of 'horses in the morning' went to a place in california where they let her ride zebras and a baby zebra/donkey (i think) was born. I could be wrong but I think it was Welcome to our website
. You can find horses in the morning on facebook or the web and ask jamie personally I suppose because it MIGHT be public.
Anyways to answer your question they can be trained and most are trained by good trainers and become better riding animals and at groundwork than 9/10 of the horses owned by most of the horse owners I've seen (out of 100s and 100s) lol.