I've not actually looked at the video, being close to a wild fox myself that visits us most nights and now, 2 or 3 others as well, I think it would be upsetting. I have heard about this fox though and the way it has adapted. It is incredible, from what I've heard about this fox.
My money would be on a birth defect, although, goodness knows.
A friend of mine has a dog, said dog jumped out of the top floor window, (2 storey), trying to jump onto a garden shed outside. It missed the shed completely and landed on a patio. Long story short, he was one lucky dog, all things considered. He broke a couple of bones but completely smashed one of his rear legs, which had to be amputated.
I've meet this dog a few times whilst walking my own and it's not bothered at all, almost keeping up with my own, 3 year old Weimaraner and they're no slouch when they get going.
Nature is amazing.