I wouldn't worry about it. Whether or not she meant it, it really makes no difference. The horse is your and you love her, what else matters?
Generally speaking, comments that are hurtful are usually said by people that don't feel comfortable with themselves (or in this situation, maybe what she owns). I wouldn't sweat over a comment like that, and because of the situation would take it as a compliment.
Sometimes jealousy also plays a big role in what is said