A Type D personality is distressed, Type D's have a negative outlook towards life and are pessimistic. A small event that is not even noticed by type B can ruin type D's day. Type D personalities are more subject to depression than other personality types.
For most of human history, horses were seen as prey to be hunted for sustenance. During the late Pleistocene, around 55,000 years ago, Neanderthals scavenged off of horse carcasses and made occasional kills of their own.
But around 4000 BC, people in Central Asia began to tame horses, to domesticate them, to eat them and to use them to carry things. It was probably the Indo-Europeans, living around the Caspian Sea in Central Asia, who first tamed horses for their own use.