I think he meant mare not horse here but I can't really be sure because here in Egypt we have these same "terms of endearment" for attractive women like (Farasa: mare, Gazal: Gazelle) although they're falling out of favor esp among younger generations and to my knowledge they are not really effective on women and I don't recall a situation where they worked except the one you cited here :)
Because in our case, it could mean that the female is attractive and has good physique esp. stature or it's implied that she has good sexual appetite/stamina and that's why it's best to avoid all these terms altogether and stick to plain and simple pickup lines :)
I don't think "mare" is that insulting either, an insult-lite at best.
Used in relation usually to being stubborn.
"come on, do your homework, you're stubborn like mare / you stubborn little mare"
EG "a right mardy mare"
Probably NSFW for language http://www.townx.org/blog/elliot/insulting-british-colloquia...