You may be able to plan into your budget the cost of replacing the horse if it dies, but you can't plan the cost of a liability action. Keep in mind that whatever coverage you place on your horse is for that horse only. If you're also trailering a friend's horse, your coverage does not protect the guest horse. As with insurance for protecting the horse, there are many variables when it comes to insuring the towing vehicle and trailer. The amount of protection for collision, which covers damages to the owner's vehicle, and the amount of the deductible, is a decision based on the vehicle's value and the owner's budget.
Liability covers damages to others that result from the accident.
Don't assume that your automobile insurance covers the trailer. Some policies restrict how far a vehicle can be towed. If you go farther, you'd need to endorse that on your policy with your agent. Besides insuring the horse, owners might also consider insuring their equipment and tack under a home owner's, farm owner's, or similar type policy. Many home owner's policies do not automatically cover horse equipment and tack, and some only cover those items as long as the tack or equipment remains on premises.