A mares "bag" or udder shouldn't feel "hard". Just before a mare is going to deliver her foal, she will "bag up" Which means that her udder will fill with milk. This can happen any where from a few days to a few hours to not until the mare delivers. (each mare is different) It should feel firm. Kinda like a balloon filled with water. If the bag is "hard" (like an apple) she could be developing mastitis, which is an infection in the udder. This is usually accopanied by a fever and decrease in appetite. The mare also will usually not like you touching her udder. If this is the case, you should call your vet immediately.