Well, there is the term Pegasister for female fans, but being a Brony isn't just for guys. I USED to be a Pegasister, when I used to watch MLP maybe once a while, I knew the names, you know, just the basic stuff. But being Brony means that you just don't WATCH MLP, you LIVE MLP. Bronies are hardcore pony-lovers. Not just for guys.
(Besides, I refer to girls as 'bro' anyway. So yknow xD)