I have to agree with Daisy25 that the old owner is being presumptuous, but perhaps she doesn't have much in the way of social skills. Although she could be under the misapprehension that because she gave you the horse for 'free' (as though initial cost were the expensive part of horse ownership!), that she still maintains some sort of claim. It's a good thing you still have the emails, but hopefully she will email back and be understanding.
Whatever you think you can do or believe you can do, begin it. Action has magic, grace, and power in it. – Goethe