Old mare getting picked on by young mare?

Are they getting enough food? Maybe you need to put some more hay out for them? That will keep them occupied for longer too. Is your older horse ill? Horses and fish pick on sick older animals. Maybe you need to exercise the younger 1 for a bit longer, burn off some energy. If they are in season she may be feeling a bit moody too, a mare supplement might help? Ask your vet to check if her cycle is causing her any problems. A companion that will keep the younger 1 busy may help, you have to be careful as you don't want 2 horses picking on her, perhaps a shetland as they make good friendly companions ? A gelding might be better as she may bully another mare. But it might make her more aggressive towards the older 1 when she comes into season if there's a male around. If she has the supplement she may not feel aggressive though.