One thing you will hear the first time you own a young horse is how important it is not to overfeed them. It is absolutely true that stuffing a young horse full of tons of grain in an attempt to keep him fat and beautiful is likely to lead to a variety of problems. We have good old epithysitis, responsible for the crippled condition of many ex-halter champions, and OCD, a common problem in fast-growing warmbloods and other sport horse types.
Here's a good primer on these kinds of nutrition-related leg problems.
Here's one of my favorite pages on how to feed young horses so that you do not have these problems. It is not good enough to assume that just because something is labeled as a "junior feed," it is the right choice for your youngster. You really need to do your research!
That said, how can you look at a young horse and know if he is ribby because he is growing or ribby because he is starving? Yearlings, particularly yearlings who will top out at 16 hands or taller, look ridiculous. There are few things fuglier than a yearling warmblood, even though the same horse will be breathtakingly beautiful at three! We all joke about hiding them behind the barn for a reason. But that's different from a yearling who is ribby, pot bellied, and has no butt. That yearling is nutritionally deficient and not getting what he needs to grow. He can and will be stunted and never reach his full height if the situation is not corrected.
Here is a classic example of the starving yearling. (Click on the pic to see a larger version) This is a Trakehner/Spotted Saddle Horse cross, so first question is of course WTF were you trying to accomplish with that nonsense cross? Second question, what are you feeding him, hay you scraped off the bottom of your neighbor's barn floor from three years ago? They claim he is UTD on deworming. I would be embarrassed to put this for sale on the Internet. His spine is sticking up and so are his hip bones, his whole rib cage is visible and he has no butt. This one is as bad as "Prince" who thankfully got rescued. He is in Illinois and the picture title should tip you off as to the ad location if you are interested.
Now, here is a ribby yearling in otherwise good condition. (Click to see a larger picture so that you can see all the details). She is butt-high and it's obvious she's in the middle of a growth spurt. You can see ribs but her butt is rounded, her spine does not show and her withers are not sticking up like a 30 year old cart horse's. There is no "worm belly" hanging down announcing the existence of parasites.
This is a yearling in correct weight, who can and did mature into a sound, healthy adult horse.
Now, here is one that is mid-rehab after being rescued from a BYB where he did not receive proper care. He has gained back some of his weight and the effects of deworming are starting to show. The belly is tucking up from where it was and his back/hip are filling out. He still needs a few pounds, but he is getting there.
While a slim neck is desirable on a stock breed pleasure horse, it should be slim because the horse is fit and well muscled, not because of a lack of flesh. For example, this horse is a healthy weight with a pretty, pleasure horse "pencil neck." She is in beautiful condition - fit and ready to show but without any sign of being tucked up/drawn up. (and what a pretty mare...you can drop that one off at my house anytime!)
Now, in case you were wondering what a grossly overfed yearling likely to be a crippled-up mess by age five looks like...HERE YA GO!
For $7000, I'd rather have a used luxury car. It will run many more years than this filly, and won't flip over on me while having a HYPP seizure.
You people breeding this stuff suck beyond words. Gross.
