The law considers your horses and other pets in the same category as assets, such as your house or automobile. However, there are options to guarantee their care, according to Newsmax in "Equestrian Legacy Planning Through Trusts."
You can now set up a trust for the benefit of your horses. You can put assets into the trust and leave directions for how those assets are to be used to take care of your horses.
A trustee can then be appointed to make sure your wishes are carried out when you are no longer around to care for your horses. This will ensure that there is a legally enforceable way for you to make sure that the horses are cared for as you would wish.
An estate planning attorney can advise you on creating an estate plan that fits your unique circumstances and including the care of horses or other pets.
Reference: Newsmax (Jan. 17, 2018) "Equestrian Legacy Planning Through Trusts."