If you think that this has anything to do with flies, please let her keep her tail! The tail is the horse's #1 defense against biting insects. I doubt that she's able to cut her legs with her tail, but if she's constantly swishing hypothetically it's possible? More than likely it's nothing and will go away after fly season is over. Have you made sure there's nothing in her pasture/stall that could be causing the scrapes? If so, I'd put some ointment/neosporin on them and not worry about it.