It is not advisable to give horses walnuts because they are considered to be poison for them. This is actually the case with many other types of foods and nuts, including peanuts and even onions.
Even though walnuts may be good and nutritious for people to eat, it isn’t necessarily the same for animals like horses as well as dogs and cats which are also adversely affected by the consumption of these nuts. Because walnuts contain so much phosphorus, they are considered to be poisonous for most animals like horses.
Feeding horses walnuts can lead to a variety of health problems, including an upset stomach as well as diarrhea and vomiting.