My experience is not with a miniature, but with a Paint horse we had adopted. Pleasure was rather old but had a great attitude and cheerful spirit right up to the end. We battled his Cushings ups and downs for six years. It is treatable, but as far as I know, not cureable. Here's some of what we learned over those years of treating and caring for him.
Many Cushings horses are insulin resistant, thankfully our boy was not. The trademark "buffalo coat" is a common indicator, as is a lack of muscle tone along the topline, making them appear thin and unthrifty. The coat is wavey or curly, longer than usual or of uneven lengths, and does not shed well. Also, lameness is common but again, and thankfully, we did not experience this.
Pleasure would periodically do what I called "crash". And by this I mean he would suddenly and dramatically lose weight. I came up with a special mix for him of several different feeds and an oil that really helped him. Cushings can also be hard on their teeth and he rapidly lost several. I was just about to experiment with Chaste Tree Berry from the natural food market when he passed on quietly one evening. Because Cushings is a disorder of the pituitary gland, chaste tree berry, crushed and given over grain had been found to help ponies with it. It normally is used for "woman problems".
Thyroid tumors can mimic Cushings and an idicator used to help determine which gland is being affected prior to testing is the horse's attitude and demeanor. With Cushings, they are usually alert and still active, although some can also be lethargic. With thyriod, they are usually depressed.
