You make it sound like sub-Saharan Africans were offered an unparalleled opportunity with horses and rejected them out of some sort of pique. A quick search indicates that endemic diseases in the region made the husbanding of horses completely impractical until they could be bred over centuries to be resistant to the diseases transmitted as easily as say... a bite from an insect.

The Pony Express lasted all of 18 months and was apparently financially unsustainable.