There's one older version of this question here which gets into a few southern organizations like the KKK, the Crescent City White League, and the Knights of the White Camelia.

There's another older one here that gets into the Wilmington Insurrection in North Carolina and the James-Younger Gang (a little bit) in Missouri.

Related question- were there any people in the South that refused to admit the war was over and kept fighting?