But would they really be beasts of burden? I mean, the Americas HAD the bison and pronghorn and musk ox, africa had Zebras, Rhinos, Wildebeasts,
What about the extinct megafauna makes them suitable for domestication? Several of the more charismatic species are too big. Penning megatherium would not be a fun activity. Of the extant members of the genus Equus, two of nine are domesticateable. Want to bet on which side of the equation the american wild horse would fall under? Sure, okay, maybe the Yukon Horse is a member of E. ferus. That's no guarantee.