The phrase "could eat a horse" is an idiom that means to be very hungry. Some antonyms for this phrase would be "full," "satisfied," "content," or "not hungry." These antonyms describe a state of being when one is not experiencing hunger and is comfortable with one's current level of food consumption. Other antonyms could include "nauseous," "uninterested," or "indifferent," which suggest a lack of appetite or an aversion to food. While the phrase "could eat a horse" may be a colorful way to express hunger, it is important to listen to one's body and only eat when genuinely hungry.