I just posted this on fb about Duncan so copied and paste in response to yorkigals question about why not?
Apart from the fact that he is hand standing everywhere, he was born wi a severe deformity, his rear legs were unable to bear weight and fused at the joint, he also has major spine issues, so they amputated and he will face further surgeries as he grows. So not only is the poor mite in for a lifetime of pain, he's going to be putting his body under permanent stress which results in everything from a suppressed immune system to depression. Then you have the unknown aspects of consequences of amputation, phantom limb pain etc, excessive stress on the heart, likely damage to the spine she to his altered walking position and so on. THEN you need to consider the social aspects - lots of dog communication is done by posture, tails and bum smelling - all of which he will not be able to do in the normal way, likely leading to doggy social problems. Then hygiene - how does he toilet? The list is exhaustive and he is just three months old - he should have been euthanised at birth, it would of been the kindest thing, but instead human selfishness takes over.
As for why we do it for humans and not animals - that's a moral question. I would without a doubt abort if my child was severely deformed to the point where quality of life would be affected. If the child was born deformed (ie it was not detected on scans) society says I have to do all I can to help that child survive. On a personal level given the choice of making my child face a life of pain and surgery or allowing a painless, dignified end at a day old - I'd take the latter option. My friends child was born deaf and blind she had a severely deformed spine and spent 12 years in hospital having operation after operation, then moved to a hospice where she died. The only sound she made was screami when they tried to straighten her spine with a metal rod and it was so painful she screamed and cried day in day out. She didn't understand what was happening, all she knew was a life of pain, so yes if that were my child I'd let nature take its course ( or I'd of euthanised if it was legal) for her sake, not mine.