Not knowing her history, I would be hesitant to change her diet immediately, and add in a medicine without testing to see what is really going on. There is a chance that doing that, and just waiting will do more harm than good. I am not sure how much it costs to do a blood test, as I've never had it done on any of my horses, but I would suggest you go that route now. Though I will add that I knew someone who had a pony, not a mini, who was okay when they got her, but then they stopped working her, and she developed a very cresty neck, and the fat deposits. Nothing was wrong with her other than they needed to cut back on her feed, and start an exercise program. Good luck with her.