Why did knights become a distinct group, and when did they decline?

During 9^th century of Europe local wars became common. There were frequent localised wars in Europe from the ninth century onwards Good cavalry was required for wars. The amateur present-soldiers were not good enough for those wars. This need was fulfilled by the knights. In this way, knights became a distinct group to achieve this aim. The fall of the feudal power resulted in the decline of the knight during the 15^th century.