I am reading Guns, Germs, and Steel by Jared Diamond. At this point in the book he is explaining why certain animals were never domesticated:
Bear meat is an expensive delicacy, grizzlies weigh up to 1,700 pounds, they are mainly vegetarians, their vegetable diet is very broad, they thrive on human garbage, and they grow relatively fast. If they would behave themselves in captivity, grizzlies would be a fabulous meat production animal. I am not aware of any adult that has been tamed.
Hippos, as four-ton vegetarians, would be great barnyard animals if they weren’t so dangerous. They kill more people each year than do any other African mammals, including lions.
Zebras have the unpleasant habit of biting a person and not letting go. They thereby injure more American zookeepers each year than do tigers! Zebras are also virtually impossible to lasso with a rope — even for cowboys who win rodeo championships by lassoing horses — because of their unfailing ability to watch the rope noose fly toward them and then to duck out of the way.