Green’s Dictionary of Slang

horse and trap n.

[rhy. sl.]

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

[UK]J. Franklyn Dict. of Rhy. Sl.
[UK]D. Powis Signs of Crime 188: Horse (and trap) Gonorrhoea (clap): ‘Got a dose of horse.’.
[UK]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)].

[UK]J. Franklyn Dict. of Rhy. Sl.
[UK]Dodson & Saczek Dict. of Cockney Rhy. Sl.