Different parts of the dog’s body “mature” at different rates & at different times. That’s why it is extremely important to study the anatomy of the parents. A little less important is the anatomy & characteristics of the grandparents. Pups go through changes as they grow to skeletal maturity, which is why the parents should be evaluated as much as the individual pups. The top line keeps changing as growth rates vary. The length of the croup increases but the angulation does not. The top line settles around 18-19 months.
Cow hocks in dogs is caused by growth-rate differences between tibia & fibula (the two bones of the lower rear leg). One “slows down” (tends to stop growing earlier than the other) and the resulting twist causes the lower part including the feet to point away from the center line more than they should. A little is normal, but too much leads to inefficient movement. Cow hocks don’t get better, but dogs sometimes learn to compensate a little when gaiting. Not enough to get away with it in the show ring.
The reason you don’t see it as much in younger pups is that the growth rates of the two bones is the same in the beginning. It shows up as the dog gets bigger & one part of the lower leg slows down while the other bone keeps growing at the same faster rate.
Wild canines almost never have cow hocks, wolves, etc. That’s because it is unsuitable for long-distance (endurance) trotting. The last wolf in the pack doesn’t get as much of the caribou (reindeer) or whatever & does not get the chance to establish dominance & the right to breed. Domestic dogs are allowed to breed regardless of physical attributes. Domestication has foiled Nature’s “natural selection”. 
To hide cow hocks
1. The handler can take the dog away from the judge in a slight curve, so the judge can’t see the rear action well.
2. Do the “down and back” at a faster speed than necessary. You might have to train the dog not to break into a gallop, in order to get away with that.
3. When posing the dog & the judge moves from the front to the back to look at its rear, don’t stretch the dog out. The further back the “hindmost” hock & the greater the distance between the two rear feet, the more obvious the cow-hocked stance or defect will be. Cow hocks can generally be less obvious if the rear legs are not positioned so far apart, nor too far behind the torso. Less “extreme”, in other words, usually makes this slight defect less obvious.
4. In stacking, to get the best view, GSDs should have the rear-most hock vertical & the toes on the other leg (the one under the torso) no further forward that the knee-cap (patella). This is the most natural stance, and if all dogs are posed the same way, the judge can make a better evaluation of the class.
~ By Dr. Gautam Srivastava
LUV SHEP Kennel, Vijaywada.
Cell: 7893009911