In Jared Diamond’s Guns Germs and Steel, he asserts that zebras cause more injuries to zoo keepers than any other animal. At least that’s how I remember it–I gave my copy away years ago and never replaced it. In any event, it’s common knowledge that zebras are mean and you can’t really tame them. Horses on the other hand are every little girl’s best friend and they are happy to wear garlands of flowers and nibble apples out of your hand. Why such a difference when on the surface zebras just seem like striped horses?
Before I go any further I should say that the rest of this post is a pure mixture of Wikipedia and armchair theorizing. You have been warned.
The answer has to be geography. The savanna is a tough place. It’s got cheetahs, hyenas, wild dogs (that also make terrible pets), crocodiles (near the water anyway) and of course lions. Friendly and laid back on the savanna equals dead. You have to be fierce (zebras/meerkats), big (elephants/hippos/rhinos/giraffes) or fast and high strung (impalas and ostriches) to survive. But what about those friendly monkeys that like to sit on shoulders in the bazaar? Don’t they live on the savanna? Well they can watch the action from the tops of trees and eat their bananas in peace. Zebras don’t have that option.
The temperate grasslands where horses evolved are a whole different story. A lot more empty space and a lot fewer predators mean horses could afford to just horse around all day before humans came along and domesticated them. Even the temperate grassland predators are basically wild house pets. Wolves are genetically identical to dogs and nothing else is big enough to give a horse more than a kick in the shin.
Of course this very reasonable explanation doesn’t change the fact that it’s grossly unfair that we can’t ride zebras around for fun–I mean that would be so cool! Maybe someday someone will splice nice genes into the zebra (or striped genes into the horse) and I’ll get to live the dream. Until then I’ll stick to the mountain bike for my riding adventures. Oh well.
Update: This is wild speculation by someone who doesn’t know jack about zebras. If you want an expert’s opinion, based on real knowledge, check out Rory Young’s post on quora.com.