Personally, I don't capitalize it in sentences, but I don't mind if others do. I kind of like the way it looks in lower case, like "I'm a brony and it ain't nothing special or weird." When differentiating between ponies in real life and ponies that are part of the Hasbro toyline, this good little read on bronies capitalized it to be "Ponies":
http://www.collectorsweekly.com/articles/my-little-pony-smackdown-girls-vs-bronies/