Animal (land mammals, in this case) skeletons have considerably more range of motion than a human's. I've highlighted the joints that rotate on the hind end of the dog skeleton that you've linked, and on a horse's skeleton. A large part of the animal world walks on their toes, not on the flat of their feet.
http://i.imgur.com/j5wM3jH.png Dog
http://i.imgur.com/BdhCsDE.png Horse
An animation of a cow skeleton walking
I've been quietly observing this thread for a few weeks now and I thought I'd give input on why bone translations and extra bones in legs and spines, etc are currently necessary for capturing realistic movement in animal avatars. Animals have much more flexible skeletons (a cheetah's spine is INCREDIBLY flexible!) and the positions of their joints are very different. For example, a knee on a horse's hind end is the joint above the tibia. Canids, equids, and felines though all move differently from each other, even though they all walk on their toes (a horse's hoof is counted as a toe, since it's a remnant from when horses HAD toes--about 30 million years ago.) So yes, the toes are a joint. Our toes are jointed, animals just have better control over them ;p
Not sure how good my explanation skills are but that's pretty much the gist of it.
Edit - revised my diagrams