Actually, "brony" is a portmanteau "bro" and "pony". The difference is only significant because bronies originally came from 4chan, where, among other uses, "bros" were people who were helpful and kind to others. The anonymity of the site meant that you never knew if a "bro" was really male, female, or anything in between, and it didn't matter to anyone who posted there; a bro was a bro, no need to complicate it any further, and thus bro became gender neutral in that culture. So, with "brony" being a combination of two gender neutral terms, I've never really understood why some feel the need to make it gender exclusive; I'd rather keep with simplicity. But, as others in this thread have said... some people prefer using pegasister instead. I'm sticking with brony, myself.