This may have happened to you--as you take your horse's blanket off you notice that his hair is disappearing on his shoulders, leaving a bald spot. If the problem is severe, it may not only be the hair that is being rubbed away, but the top layers of skin.
The major cause of horse blankets rubbing is a less-than-perfect fitting blanket. The neck of the blanket may be cut too small for the horse, causing tightness around the shoulders, especially as he moves around his pasture. Believe it or not, blankets that are too big for the horse can also cause rubs. I had this problem with Annapolis a few years ago. The horse blanket I got for him was too long and the excess length (and therefore weight) at the tail dragged the whole thing backwards, making it pull at the shoulders. In addition to making him reluctant to walk forward, it also rubbed the hair off on his shoulders.
Always Measure Your Horse
The first line of defense is to make sure that the blanket you purchase, whether it is "off the peg" or custom made, fits your horse well. In my previous features about stable and turnout blankets I gave a simple reference guide to fitting horse blankets. This should only be considered a rough guide however. Horses are made in all different shapes and sizes. Two 16 hand horses may be considerably different in build and require different sized blankets. A few winters ago I invested in a custom-made blanket for Annapolis. He was measured from withers to tail, from chest to buttock and from chest to withers (to get the size of the neck-opening) The result has been a well-fitting rug that Annapolis can walk comfortably in and does not pull across his shoulders.
Many blanket manufacturers now line their horse blankets with nylon panels in the chest area to alleviate rubbing. This can be very effective as the nylon slides across the hair as the horse moves, instead of catching and pulling at it. Other remedies are the various makes of "horse underwear" now available.
These items of horse clothing go on, of course, underneath the blanket. They may be made of a stretchy fabric, attaching with velcro closures behind the forelegs or some are available which have a bib design. Whichever design you select, making sure that it fits well and keeping it clean will make it even more efficient in protecting your horse's coat.