Fetlock boots are a form of horse boot designed to protect your horse against injuries caused by brushing, i.e. touching one leg with another. If your horse is prone to brushing, fetlock boots are a good solution.
Fetlock boots are reinforced on the inside to protect the fetlock joint, and they are more flexible than standard horse boots. The boots are also available in synthetic material, as the wear is lowest here, but due to certain material mixtures they are more flexible than, for example, common hard-shell boots.