Horses and pigs are known to have good appetites. Therefore when you say that someone eats like a horse or a pig, what you are implying is that the individual consumes a lot of food. Careful users of the language maintain a subtle distinction between the two expressions. When you say that someone eats like a pig, it has a negative connotation. Pigs are supposed to be rather noisy eaters. They slurp, grunt, and drool when they eat. I am sure other animals do the same thing, but unfortunately for the pig, it is always associated with these things. Therefore when you say, `He eats like a pig’, what you mean is that he eats a lot, and has very bad table manners.