Typically refers to 13-30 year old female fans of the 2010 reboot of the show My Little Pony: Friendship is Magic. Similar as, but not to be confused with: "brony" (bro+pony) which is more often used for the male equivalent. It is a well-known and undisputed fact that bronies are on average about...20% more awesome due to their rarity.
ex. "You wouldn't think she would still watch that show at her age, but she's obsessed. She's a total filly!"