Short answers, pulled from the story.
The first archaeological evidence of horses used in warfare dates from between 4000 and 3000 BC in the steppes of Eurasia, in what is now Ukraine, Hungary, and Romania. The first recorded depiction of horses pulling a vehicle in battle is the Standard of Ur, from Sumer, dated around 2500 BC.
The oldest known manual on training horses for chariot warfare was written around 1350 BC by the Hittite horsemaster Kikkuli. A separate early text on training riding horses for cavalry, titled Hippike (On Horsemanship), was written around 360 BC by the Greek cavalry officer Xenophon.
The first set of paired stirrups appeared in China around 322 AD during the Jin dynasty. Stirrup technology spread from Central Asia to Europe by the 7th century, carried largely by Central Asian invaders; widespread use in northern Europe and England is credited to the Vikings in the 9th and 10th centuries.
Medieval European armies used three primary war horse types: the destrier, the courser, and the rouncey, which differed in size and purpose. Destriers were moderately larger, built for armoured knights, and distinct from true draught horses. The generic term charger was used interchangeably with all three.
The German Army used around 2.75 million horses in World War II, more than it had used in World War I, because its factories were committed to producing tanks and aircraft rather than motorised transport. The Soviet Union used 3.5 million horses during the same conflict.
Operational Detachment Alpha 595 teams were inserted into Afghanistan on the 19th of October 2001 at the beginning of Operation Enduring Freedom, making them the first U.S. soldiers to ride horses into battle since the 16th of January 1942. Horses were the only suitable means of transport in the mountainous terrain of northern Afghanistan.