Green’s Dictionary of Slang

pretty horse breaker n.

also horsebreaker
[the way such women showed themselves off in a horse and trap, mixing with society as it too paraded in Hyde Park in London and similar places. Their riding costume was often known as abandoned habits n.]

1. [mid-19C] a high-class prostitute; orig. a woman hired to ride in Hyde Park.

2. attrib. use of sense 1.