Knights still fought past the end of the 'Middle Ages', however they became less and less effective against the lighter horse and foot soldiers armed with long range firepower able to penetrate even the heavy armor of the 'modern' knight.
Around 1494 horses were re-introduced by the Spanish into the Americas (archaeological evidence maintains the horses first appeared on this continent and then vanished). Cortes and his bunch overran the natives of South America, thoroughly startling them with the famous Iberian War Horses around 1519, and the history of the Americas was off and running with the use of 'modern' cavalry in its wars.
Many 17th century cavalry leaders rode the handsome Friesian horse of earlier warfare. Just before world war I, crossing with the Oldenburg horse saved this precious Friesian breed from extinction. During world war II, because of petrol shortages, the Friesian breed came into its own again.
With hand to hand fighting becoming obsolete and the battle tactics going more to the planning table than the talented and experienced warrior, a different type of horse was introduced into the fray to not only pull the heavier equipment but to mount general troops (most of whom were not the advanced horsemen of earlier warriors). The 'cold blooded' draft breeds were combined with the 'hot blooded' war mount breeds to form 'warm blooded' horses. The heaviest of the German Warmbloods was the Oldenburg established in the 1600s.
Photo from: http://members.tripod.com/~White_Arabian/oldenburg.html
There were (and are) many other breeds of warm-blooded horses and they were bred for a placid temperament, size, and durability. More maneuverable than their cold blooded forebears and much more calm than their hot blooded ancestors, these horses were heavily depended on to pull the heavy artillery and other equipment needed and to transport freshly trained troops whose riding ability was not that of the ancient mounted warrior. In these later wars, many more warmbloods were in battle, serving with distinction and honor, than the talented warrior horses of earlier times.
Even though the civil war in the United States of America used heavy artillery, cavalry was strongly depended upon for fighting in this war. Thoroughly ignoring the lessons of history which made fully armored knights obsolete, unarmored idiots..., uh, soldiers on horseback, with lance and saber, charged artillery positions with reckless abandon.
The lessons of this kind of combat were again somehow lost by World War I through the start of World War II and horses were still considered indispensable for fighting as well as being used for drawing heavy artillery through mud and across long distances. Any equine, trained or untrained, was eligible for conscript into these wars, or indeed, any other battle going on at this time. While human war heroes were decorated with ribbons and medals, horses surviving the World Wars were usually sold for meat and butchered most cruelly, many having been injured in the line of duty and deemed 'unsound' for further use. The last sad act in the drama of horse cavalry was the charge of the Polish lancers against German tanks and dive bombers at the beginning of World War II. (Encyclopedia Americana)
Horses loosed from the despicable 'Cowboys and Indians' game in America banded together and formed wild herds, surviving quite nicely until entire herds were stampeded off high cliffs and their remains sold for dog food, glue and hides.
LET US REMEMBER WITH LOVE AND GRATITUDE ALL THOSE HORSES WHO SERVED US WITH VALOR AND DEVOTION TO DUTY, MOST WITH THEIR VERY LIVES DURING THIS TIME IN HISTORY!
All dates given are from the Encyclopedia Americana
Music is an excerpt from "The Fields Of Anthenry" from James Galway's CD "The Celtic Minstrel" (RCA Victor)
Title clip art is from: http://www.geocities.com/Heartland/Flats/1592