File Alexander Sarcophagus jpg Look how small these ancient horses really were Note knees of riders head of riders Selective breeding is an amazing thing I suppose that basically the horse was of greater benefit as a riding and harness animal by getting a little larger adding either strength or speed or both while bovines were a bit easier to handle if they got smaller Donkeys weren t ridden so much as they were used as pack animals so humans would probably lack the motive to breed them taller other than arguably jacks for siring mules But as for local horse domestication originates from the Ukranian steppes and spreads out from there and if you want to get technical the animal itself is native to steppe country in both Eurasia and North America So while various indigenous wild equines eventually scattered themselves about the domesticated ones probably got bigger before they got local genes into them and in Northern Europe most of the obviously landrace types actually rather small Just saying We don t start seeing stuff routinely over 15 hands until well into the late Middle ages if not later Even accounting for bad art File Charlemagne and Pope Adrian I jpg File Einhard vita karoli 13th cent jpg File Paolo Uccello 023 horses clearly took a while to get up to modern sizes talk