Check the fencing. Horses have the uncanny ability to find anything to hurt themselves on. Remove barbed wire from any fencing that horses come in contact with. Barbed wire can severely damage and even kill a horse if it tries to run through it or gets tangled up in it. Make sure there is plenty of high, dry ground during the rainy season.
Horses standing in mud or water can damage their hooves and legs.
Check the local county regulations about the number of horses you can keep per acre. Typically, one acre can sustain two horses. Make a regular check of the fencing to make sure there are no damaged or missing boards that a horse could escape through. Consider removing excess manure with a large backhoe to keep flies at a minimum and to prevent reabsorption of expelled larvae. Rotate pasture, if possible, to prevent overgrazing.