I give a liquid horse supplement as well, Nutri Aid GCM this contains Glucosamine HCL, Chondroitin Sulphate, MSM, Vitamin C, Mineral Manganese and devils claw. My dogs are doing fantastic on this supplement, plus being liquid it's very easy to put in their meals. Having 5 large dogs it's also more economical :)
When choosing a Glucosamine supplement it is perfectly fine to give human supplements, just make sure they either contain or you give Vitamin C or manganese along with them. A dogs system needs the vitamin C or manganese to help uptake the Glucosamine into their system.
I always choose a product that contains Glucosamine and Chondroitin here is a breif explanation of how they work together.
Glucosamine provides the building blocks to synthesize new cartilage. Glucosamine is the building block necessary for the production of the substances called glycosaminoglycans. The glycosaminoglycans are combined with hyaluronic acid to make the substance proteoglycans. The proteoglycans and collagen are the main structures of cartilage.
Chondroitin also is one of the products necessary for the synthesis of glycosaminoglycans. But it probably plays a more important role by combating and neutralizing destructive enzymes in the joint. There is always a low level of destructive enzymes found in the joint, but when injury or abnormal wear occurs, the destructive enzymes and agents increase accelerating cartilage destruction. When chondroitin is added to the diet it helps to reduce the level of these destructive enzymes.