It depends. For a pharmaceutical treatment, they will inject it, adjust dosage for the weight and type of animal, and observe the outcome. If it’s cancer, see how large the tumor is.

If it’s a medical device, a large animal model is used. What animal is very dependent on the part of body you are studying. Cardiovascular tends to be a pig or sheep or dog model. The harder part is giving it a disease, since heart disease tends to develop after decades of life span and poor QoL, so Doctors and researchers try to find a way to restrict blood flow, increase heart rate to induce heart failure, inject chemicals that kills the muscle cells. There’s no perfect way but a lot of money goes into developing a method.