Green’s Dictionary of Slang

1. in sexual contexts, playing on ride v. (1a)

2. (US Und.) one who collects protection money and passes it on to gangsters, corrupt police etc.

1929		Hostetter & Beesley It’s a Racket! 228: horseman—One who collects money, for ‘protection’ or ‘immunity,’ from owners or inmates of illegal establishments such as ‘blind pigs,’ gambling places, ‘beer flats,’ houses of prostitution, etc. The ‘horseman’ gets a small percentage of the money for collecting it, the rest going to the police or political officials who are furnishing the ‘protection.’.
1950		Goldin et al. DAUL 102/1: Horseman. (Prohibition era) A collector for venal police and Federal agents of protection money from speakeasy establishments and illegal stills.

3. (US Und.) a corrupt police officer.

1949

Monteleone Criminal Sl. (rev. edn).

4. (US campus) a habitual cheat, one who ‘rides’ on the efforts of their fellow students.

1969

Current Sl. III:3.

5. (Can. Und.) a member of the Royal Canadian Mounted Police (RCMP).

1978

R. Caron Go-Boy! 143: Billy [...] walked with a terrible limp, having stopped a Horseman’s bullet during a bank robbery.