I believe many of the Santa Ana Mtn trails were game-indian-hunter trails first and adapted for rancher then recreation use. But probably each one has it's own story, just like the Laguna area trails (sure some were cut in by the rads).
BGR!!! FUUSFS!!! "Never try to outrun a ranger on a horse uphill"... "If you get caught remember: you were just riding along and met these guys, and thought this was a legit trail You don't know any of us..." Uh oh. I've said too much black helicopters... BLACK HELICOPTERS!!!
On a work crew with SHARE/IMBA one time I asked which trails they had made and they said Missing Link was done by them about ten years ago.
Mike Troy (grannygear here on STR) wrote/co-wrote a few books on the subject and had a big part in popularizing many of our SoCal trails.