When the horse in the Hanukkah story, "The Horse That Wouldn't Eat Latkes," refuses to eat latkes, it is fed oats instead. Latkes are traditional potato pancakes served by Ashkenazi Jews at Hanukkah celebrations. Latkes are not considered standard horse feed.
The horse in the latke story is fictional and an invention of the Brooklyn-based Tzivos Hashem group, whose aim is to teach children about Jewish customs and encourage religious observance. The story is not meant to be taken as a historical account or as veterinary advice.
Latkes are grated potatoes formed into patties and fried in vegetable oil. They are associated with Hanukkah because the cooking oil in which they are fried references the lamp oil in the Hanukkah story. Horses on a natural diet of grass and forage consume very little dietary fat, so consuming a fried potato would be well outside a horse's gastronomic range. However, horses do find oil palatable and can consume a diet of up to 20 percent fat safely.