Mongol warriors in the 12th century were said to have ten horses each when they invaded Europe, grass fed. Europeans fed their horses grain. London in 1900 was said to have 600.000 roaming it's streets on any day. Sorry, can't recall date, but would guess pre 2000 BC, Pharos legs bones, when examined, were curved from riding donkeys... 500 years later horse driven chariots are all over the place. The horses usefulness greatly increased when use of a harness that did not constrict their windpipe was invented.