Well, as Flocc and Howery have been saying, agriculturally this should have little (old world) impact; since cows and water buffalo are just as good at plowing. Donkeys would be used much more in the old world as pack animals, along with cattle drawn carts and other more limited animals (elephants, camels).
The real impact of the lack of horses is that there won't be any steppe invaders. There will be people on the steppe, but they;ll be more like the pre-Columbian great plains people, with small groups that are significantly outnumbered by agricultural peoples, and have no particular military advantage. Even with the advent of domesticated sheep and goats, which will presumably make the pastoral, without the benefit of the horse there will be less of them (they can cover less distance as a whole group, including the young, injured, and old); and they won't be able to make such sweeping advances on the the agricultural peoples.
The first major effect of this is that there won't be any Proto-Indo-European expansion (if, as most now do, you hold to the steppe theory) since they won't have their horse based advantage. This is the first of the a series of steppe invasion in OTL that will never happen in ATL. Ever. So, agricultural peoples will have much less to worry about.