Many people find horse insurance confusing because they are trying to think of it like they do their own personal health or life insurance. It has some similarities but also some big differences.
First, everything starts with the mortality coverage. Before you can add medical or surgical coverage, you must first be insuring the horse against death. Some mortality policies also include coverage for theft within this. Mortality coverage is effectively life insurance on your horse. Premiums are high as weanlings & yearlings, drop down from age 2 to typically age 14, and then start increasing at age 15 & beyond. Usually around age 18 or so, the carrier will want a vet exam stating that the horse is still healthy before they will renew the coverage. Even though some horses are known to live well into their 30s, the average mortality table on a horse is typically mid-20s and that's why the insurance premium increases at age 15. Imagine the cost difference for life insurance of insuring a 30 year old vs a 70 year old.
Once you have mortality insurance, you can add on a host of endorsements - surgical, medical, loss of use, or stallion accident/sickness/disease. The most common is medical so that's all I'm discussing here. Some carriers require that you insure the horse for at least the same amount of the medical insurance that you wish to carry while others only deal with a minimum policy premium. Medical insurance is different than your own health insurance in that it does not cover vet farm calls, any routine medical treatments (vaccines, teeth floating, etc.), nor anything not curative in nature (no chiropractic or massage for example). Equine medical insurance is designed for the unexpected accident/injury/illness. It is also different in that while we call it a renewal each year, and most carriers keep the same policy number, it really is looked at like a brand new policy each year.
What do I mean by this? It means that every year they are taking a look at the health of your horse and what they will or will not cover - they will exclude anything that is considered pre-existing at the renewal date. For example, if you have a sprained knee in December and your health insurance renews in January, your new policy will continue to provide coverage for your doctor visits on your sprained knee. But if your horse has a sprained knee in December, when the policy renews in January it will have an exclusion for any new claim arising out of that sprained knee. Once your horse has been treated and a vet can sign off that the horse is sound with no lingering issues from the sprain, many carriers will remove the exclusion (if not mid-term then at renewal). But if during the course of treating the sprain they discover arthritis, then the exclusion will involve arthritis and that exclusion will not be removed because arthritis is managed and not cured. So anything that is ongoing will be excluded at renewal. If that condition can be fully treated & cured, that exclusion can be removed. But anything that cannot be cured will continue as an exclusion.
Therefore it is important to be specific about the injuries to your horse. My experience has been that the carrier tries to be specific with their exclusion. If you have a right hind sprain in the fetlock joint, they will want to exclude a claim regarding the right hind fetlock joint (and typically they will word it about claims arising out of the sprain so if something else happened - say a fracture after the sprain & unrelated to the sprain, it would be covered).
Horse insurance really is about peace of mind - you have coverage to replace your horse should the worst happen and for those that carry medical, you have peace of mind that you have some coverage against those unexpected big vet bills. And if you are lucky, you will end up just paying premium and your horse will stay safe & sound!