Depending on how much you have to lose, the few hundred dollars an attorney will charge to help you draft the agreement may be well worth it.
If the horse is involved in any kind of accident (random child feeds horse a finger while at a horse show? lessee falls off and breaks a leg?) then as the owner you’ll likely be dragged into the resulting mess.
Getting the liability wording just right for your state is not something I’d want to attempt on my own.
YMMV. If you’re a college student with no assets just loaning out your horse, you might not have as much to worry about as a homeowner or someone with retirement savings to protect.