From Old English 'hengest' (stallion) — originally a horse groom, degrading through 'trusted attendant' to 'criminal associate.'
A faithful follower or political supporter, especially one prepared to engage in crime or dishonest practices.
From Middle English 'hengestman' (horse attendant, groom), from Old English 'hengest' (stallion, horse) + 'man.' A henchman was originally a squire who attended to a lord's horses. The meaning shifted to 'trusted attendant' then to 'political crony' then to 'criminal associate.' Key roots: hengest (Old English: "stallion, horse").
A villain's 'henchman' is a horse groom. Old English 'hengest' (stallion) gives us 'henchman' (horse-man/groom). The legendary Saxon warrior Hengist who invaded Britain was also named after a horse — 'Hengist and Horsa' means 'Stallion and Horse.' The word went from caring for horses to attending a lord to doing a mob boss's dirty work — from the stable to the criminal underworld in five centuries.