Short answers, pulled from the story.
The first verified evidence of true cavalry appears on the 13th century BC battlefield of Tollense valley in northern Germany, where horse bones were found alongside human remains. This discovery marks the dawn of a new era in warfare distinct from the earlier use of light chariots by the Sintashta-Petrovka culture.
The most devastating cavalry forces emerged from the nomadic tribes of the Eurasian Steppe, including the Huns of Attila and the later Mongol armies. These groups mastered the art of horse archery and mobility to conquer vast territories, with the Parthians and Sarmatians utilizing the Parthian shot tactic.
Heavy cavalry rose to become the dominant force on the European battlefield from the late 10th century onwards when heavily armed horsemen known as milites or knights emerged. This class of aristocratic warriors was considered the ultimate in heavy cavalry and led with the lance in battle in a full-gallop, close-formation knightly charge.
The greatest cavalry charge of modern history was at the 1807 Battle of Eylau when the entire 11,000-strong French cavalry reserve launched a huge charge on and through the Russian infantry lines. This charge was led by Joachim Murat and demonstrated the devastating power of massed cavalry before infantry squares and artillery became effective counters.
By early 1915 most cavalry units were no longer seeing front line action on the Western Front due to the combination of barbed wire, uneven muddy terrain, machine guns and rapid fire rifles. The last massed cavalry encounter on the Eastern Front occurred on the 21st of August 1914 in the Battle of Jaroslawice.
Modern cavalry units are defined as units continuing to fulfill traditional light cavalry roles by employing fast armored cars, light tanks, and infantry fighting vehicles instead of horses. Air cavalry units employ helicopters while most horse-mounted units in modern armies serve in purely ceremonial roles or as mounted infantry in difficult terrain.