Green’s Dictionary of Slang

1. venereal disease, spec. gonorrhoea [= clap n.].

1960		J. Franklyn Dict. of Rhy. Sl.
1977		D. Powis Signs of Crime 188: Horse (and trap) Gonorrhoea (clap): ‘Got a dose of horse.’.
2003		B. Dark Dirty Cockney Rhy. Sl. 65: I used to sleep around but then I got a dose of the horse.

2. excrement [= crap n.¹ (2)].

1960		J. Franklyn Dict. of Rhy. Sl.
1972		Dodson & Saczek Dict. of Cockney Rhy. Sl.