it is a concealed word that deals with sex servers who stand on city corners. Used in the presence of little kids as a reference of the word "whore" in plural, sounds likehorse.
At the jail, most of the female offenders have been working on the street as Ponies.
A girl that has sex with every male she comes in contact with, including all her boyfriends friends. This is a girl who compensates looking like an ass by spreading her legs.