I'm happy that you rescued her (good for you for doing so!) but am concerned that you would not know the answer to that question.
Look at any healthy horse and compare. There should be no bones protruding, just as in a healthy human or dog. For her though it's not just her weight but her condition, her coat is dull and that's a sure sign that she's missing nutrients.
Has the grass in your yard, that you're letting her eat, been sprayed for anything?
PS she looks much older than 12 to me, the horse in my user profile (to the left) was about 11 or 12 at the time of that picture and he looks WAY younger than your horse.
Last edited by EponaLynn; 02-15-2015 at 12:26 PM.