It's been more than a decade since I have read it, but I remember Diamond writing specifically about the relative lack of domesticable animals in sub-Saharan Africa. Mali, Songhai, and Oyo didn't "[adapt] horses to tropical conditions", they imported already domesticated animals from elsewhere (North Africa). It is telling that none of your examples are in Central or Southern Africa.
Also, domestication is the process of breeding animals over multiple generations to make them more pliable and is not the same as training individual animals to pull a carriage. The very first sentence of the paragraph that you quoted from the Wikipedia article on zebras is "Attempts to domesticate zebras were largely unsuccessful."
Go take a look at where Kenya is located on a map and where Donkey's were first domesticated and tell me that I didn't cover sub-Saharan africa again with a straight face.
That first sentence "attempts were largely unsuccessful" is a cope from wikipedia editors who are mad that it keeps being brought up by race realists who are using this blurb to justify their own racism. Consider that it was a literal Rothschild who did the most recent almost-domestication. 4chan is on this like fly's are on horses (but not Zebra's, or horses painted like Zebras - https://www.nature.com/articles/s41598-022-22333-7)
The part I quoted without the unnecessary pre-thinking done with that first sentence is all I need. It was quite possible to get most of the way there over short periods of time with money and effort.