Leonard Slye, or Len Slye as many knew him then, had a successful music career that had started with the Sons of the Pioneers in the early 1930's.  Len made his first film appearance in 1935, usually appearing as a singing cowboy and billed as Leonard Slye.  In 1938, a replacement was necessary for Gene Autry in Under Western Stars,  Leonard was going to get the job but Republic Pictures changed his name...after a suggestion by none other than Will Rogers (yeah that Will Rogers, never met a man he didn't like Will Rogers).  Leonard Slye became Roy Rogers.  And, Happy Trails to you!