I've grown up in a house where I've been taught this. I've been taught there might be an underlying issue such as a thyroid problem or a dental issue.
I'm not saying each and every pony needs to be shiny, and fat but I should not be able to see your horses ribs. I should not be able to see his hip bones and emaciated withers.
I'm sick of people saying/posting that they have a thin horse and they're sorry. You're not, because if you were you'd be talking to a vet, or a horse nutritionist. You'd be having your hay and soil analysed. You'd be doing everything possible to get it in health. You'd be rugging your horse and keeping it warm. You wouldn't be excessively exercising it.
I'm sorry for the rant :( But this needs to be posted.