Yes a person, by a mass opinion can call themselves whatever in this world they want. You can call yourself a 'Girl' And be a 'Bronie / Brony'. It doesn't depend on which gender you are. Like people with boats, most people call them a 'her'. Most other people call them a 'him'. Though, whichever rolls of the tongue best is most preferred.
I find that the word Pegasister is rather self indulget to the most feminine, of the 'Fandom' of My Little Pony: Friendship Is Magic. Or whatever the hell we want to call it.
Though if you want to be selective you can even choose the reason why they are here. Artwork could get you called Pegasister, not declaring gender could make you a Brony. Writing, Pegasister, Artwork (I am aware i said this.) Brony. The list goes on. But to cut this to a short.
If you are, say a 'Feminine' being, Feminine meaning you are, as most of the most Masculine would say, 'Womanish' (This is just me.) You can call yourself a, 'Pegasister', or something along that line, what you prefer. And if you are Masculine you can be called a, if you wish 'Brony' Then again, be whatever you want. Your able to think fully intellectually, aren't you? Well?
And besides, aren't we all sort of Pegasisters? This show has mainly female characters, themes and the like. Then again aren't we all sort of Bronies? We appreciate the animation, the story, besides, who doesn't like the opposite gender a bit? If you answered homosexual you are wrong, they like the opposite gender as friends.
You Homophobe.