A Red Leg is a specific term, not a generalized or generic one.
It pertains to a group of men who were from the Atchison, Kansas area. They were organized by General James G. Blunt at the behest of Kansas Governor Thomas Carney and Kansas Senator Jim Lane. Governor Carney personally financed this armed group of Red Legs.
As a side-note adding to historical fact and actuality - Blunt, was well known to ride the coattails of Senator Lane. Regardless of his so called "accomplishments", his downfall came through acts of corruption and womanizing. Blunt died in an insane asylum due to syphilis, in 1881.
U.S. Senator Jim Lane organized his Kansas Redlegs to be a group of guerrilla fighters for the anti-slavery cause. He had a deep hatred of pro-slave Missouri. His Redlegs got their name from the red leather leggings the men wore. At first the Redlegs had the blessing of the Federal government and were paid by the government.
Redlegs were a band of band of militants who refused to join units officially sanctioned by the U.S. Army. These so called "men" terrorized the Kansas-Missouri border, and which in turn incurred the wrath of Quantrill, Anderson, Todd, etc.
It has been said that the Civil War actually started in Kansas, especially along the Missouri-Kansas border.
Osceola, Missouri, a town of some 2,000 inhabitants, was a known pro-slavery haven for Southern sympathizers.
1861 (September 22) -- Lane and his Redlegs attacked and destroyed Osceola, killing nine Southern sympathizers. Anything of value was taken back to Kansas, including 360 horses, 400 head of cattle, and 200 slaves.
This attack on Osceola was the justification used by Quantrill and his men for the sacking of Lawrence, Kansas.
Some of the men which comprised the Red Legs were "Wild Bill" Hickok and William S. Tough.
Matter of fact, William Tough was known even around his own men as "He is pure horse thief and murderer."
Something a Red Leg could be proud of !