Cleopatra is the person who tends to come to mind whenever someone says "female Egyptian ruler," but we're of the mind that Hatshepsut needs more love. She's generally regarded as one of ancient Egypt's greatest Pharaohs, though nowadays she's known less for her accomplishments than the fact that she ruled as a male. Well, kind of. Being Pharaoh was a dude's job back then—being a man was just part and parcel of the identity. There wasn't even a word for a Queen who ruled as opposed to one who was married to the King. So, while Hatshepsut maintained her female identity on a day-to-day basis, on monuments she's King Hatshepsut, rockin' the traditional false beard.
You can go all-out for the costume like Take Back Halloween does, but this list is for the procrastinators. Get a white dress, a Pharoah's headdress from any Halloween store, and some sandals, and you'll be good to go. Given how early stores start selling Christmas decorations, I'd think you might even be able to find a large decorative candy cane that you could repaint as part of your crook and flail.
Another possibility: Go as King Hatshepsut, monument version, and fake beard it up. It'll give people who think you're Cleopatra pause, and it has the added benefit of keeping your face warm on a chilly October night. —Rebecca
(picture by Steve F-E-Cameron, via Wikimedia Commons)