i think i must agree with arylett's argument that "pony fan" is the only actually gender neutral term for this particular fandom. being from rural texas i would be uncomfortable referring to any female as a "mare" because it is dangerously close to the term "hag" meaning an old, infertile and typically inhospitable female horse. the reason i say "dangerously close" is because women in my neck of the woods generally are safest assumed to be dangerous, it's a simple survival rule.
in conlusion i think "pony fan" is really the best gender neutral term to use, as the key demographic of the show is young girls, and typically grown women have no problem finding appreciation for what is intended for their younger counterparts. "brony" really in my thinking is a masculine term used to refer to the entertainment industry's equivalent of the fishing industry's term "side catch" meaning fish caught in the net outside of the target species. we are men who have been caught up in the magic of pony friendship, we are the exception to the rule, we males are specifically the bronies, women do not really fit this classification.
that is my thoughts anyhow