The word "henchman" has appeared in the name of Hengest (Hengist and Horsa; legendary brothers said to have led the Angles, Saxons and Jutes in their invasion of Britain in the 5th century), the Saxon chieftain, which was often rendered as Henxman in medieval English, then the word became universally used in the middle of the 18th century.
The modern sense of "obedient or unscrupulous follower" is first recorded 1839, probably based on a misunderstanding of the word as used by Scott, and is often used to describe an out-and-out adherent or partisan, ready to do anything. 
