Lucky he got them confused! In the UK at least, calling someone a horse doesn't really make sense, but calling someone a mare is fairly insulting.
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 :)
EG "a right mardy mare"
Probably NSFW for language http://www.townx.org/blog/elliot/insulting-british-colloquia...
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"