The Equestrian Royal Guard is a predominately male organization. However, there could be quite a few females in the Earth Unicorn and Pegasus Guards (EUP Guards, an alternate name for the Royal Guards).
For example, could Rainbow Dash have an older sister who serves in the Royal Guard? Is Scootaloo's mother a Royal Guard?
I suspect that MLP season 7 will reveal that there are mares in the stallion-dominated organization, as Princesses Celestia and Luna decide that Twilight Sparkle, as an Alicorn Princess, should have some EUP Guards under her direct command.
Scootaloo's parents, in this vein, both serve as Royal Guards, and are part of Twilight's Guards since their home is in Ponyville, and they want to keep a closer eye on their daughter.
Moreover, RD's older sister also serves as a Royal Guard, and, too, is assigned to serve under Princess Twilight, and there is sibling rivalry between the Dash sisters.