We generally figure a horse will eat up to three times as a cow - but we don't give them any grain usually. Cows are much more efficient in food conversion.
And yes, a horse will eat 24 x 7 if they get the chance. And yes, a cow eats with its tongue, a horse with its teeth. Sheep also eat with their teeth, which is why after sheep graze, there is nothing left for a cow.
And if a horse gets to eat all day long, without exercise, they will get fat, which they have in common with people. Mules on the other hand seem to be more intelligent than either.