A fictional character generally portrayed with red hair, used to euphemize that a woman is on her period/is menstruating: "Aunt Flo is visiting." Usually leads to unimaginative humor about how since Aunt Flo is in town, one will end up sleeping on the couch, or have to watch their step so they don't piss her off, et cetera.

And of course, to completely flog this deceased equine: Aunt Flo visits at that time of the month.