Is it wise to ride a horse with Cushing's disease? What should be the cut-off sign not to ride him?

You should first consult with your veterinarian before beginning an exercise or riding program. However, generally speaking, horses with Cushing’s can be ridden like any other horse, particularly if the Cushing’s syndrome is well controlled with medication. Still, it is wise to consider just a few precautions:
1. Overheating: If the affected horse has not shed out or has grown its winter hair coat too soon in the fall, a body clip may be needed to keep the horse cool during exercise. Appropriate medical treatment for the disease will also facilitate more normal and timely hair coat growth.
2. Laminitis: If the Cushing’s patient has foundered, it can be ridden so long as the founder has been addressed and the horse is comfortable on its feet.
3. Vision: A few horses with very advanced Cushing’s can develop blindness due to impingement of the pituitary on the optic nerves. If there is any question about the horse’s vision, a veterinarian should be asked to verify its vision status. Well-trained horses with vision loss can very occasionally still be used for limited riding on very smooth surfaces, at a walk, such as in a therapeutic riding program, with a person leading the horse.