Eat a horse
Although a citation of the expression 'I'm so hungry I could eat a horse behind the saddle' has been found from the late 1600s, it is no longer in current usage. These days, when somone says, 'I'm so hungry, I could eat a horse' they are really confusing two separate sayings, namely, 'I'm as hungry as a horse' and 'to eat like a horse'. Both these latter two expressions refer to the large amounts of food that horses can consume. Please refer to the correct sayings, Eat like a horse and Hungry as a horse.