An overly passionate person, concerning 'my little pony', who spends all their time: talking about it; having deep discussions about it; constantly making photos of it and, whenever someone criticises the show or anything to do with it (whether it be for a joke or not) they contribute to arguing with the person. Eventually this degenerates into passionate, one sided, discussions; trolling the person (who made the argument in the first place) and ganging up on him.
Essentially, it's someone who tries to enforce 'my little pony' outside brony communities e.g. groups which have nothing to do with it or people who aren't interested.
This is different from people who likes 'my little pony' in general and watches the show, and discusses amongst other bronies.
Person 2: You said it, bronies unite! Bro-hooves!
Person 3: No one cares, stop being a branzy