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.
The natural diet of the horse is grass. Grass contains most of the nutrition a horse requires to be healthy. It also contains silica, which is important for dental health. Primitive horses can live on sparse rations and often have to make do with less than ideal pasture and living conditions.