Depends on the type of insurance.
For Mortality: Age, breed, use and value of the horse.
For Liability: number of horses for each type of exposure (number boarded, number in training, number of school horses, number of students etc) and you will usually need the receipts generated for each exposure.
For Farm: List of dwellings, barns and other structures. Type of construction, age of buildings, square footage, type and age of roof. List of tractors, tack, tools and other equipment. For equipment you will need year, make model and serial numbers (Usually not needed to obtain a quote). REMEMBER: If it not listed on the policy, it is not covered! You will also need to provide the liability information as well.
For Auto: List of vehicles, VINs for each, limits needed, deductibles, list of drivers including driver’s license and date of birth.