I would like to respond more to BabySnow's post. Just things to think about.
Point #1: Well, your guinea pig may enjoy it, that doesn't make it something they need or good for them. Think of all the foods WE enjoy but are of no value otherwise and are just fat and calories, would you have your children live on McDonalds just because they enjoy it? A lot of those pet shop piggie treat things are like that. Think of all the junk cereals that are like fortified with essential vitamins blahblah. Well the cereal being full of sugar kind of undoes that. Like, cake, ice cream, doughnuts,and the like. Yes, we enjoy these but are they good for us? No way!
Point #2: As Ly mentioned Guinea Pigs are herbivores. Their bodies are not made to digest meat, grains, dairy, and the like. For them to be healthy you should only feed what their bodies were designed to digest.
Point #3: Horses and Guinea pigs are different in many ways, different in size, different anatomically and so on. I know many people say an animal is an animal is an animal but this just isn't the case. Just like no 2 of us are JUST a like. For example, one may have special diet needs because they are diabetic or so on, and another person may be normal. No 2 creatures are exactly alike.
Point #4: These things are a waste of your money that you work to get as well. And are mainly useless gimmicks for pet shops to just try to sell you extra stuff you don't really need.