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After this fall, he has developed a skin hardening about 5 x 5 cm size on his back. What can it be?
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When was the fall? If it has only been a matter of hours, it could simply be muscle bruising, just as we humans get.

Does he show any signs of pain when you touch the area?

I would keep an eye on him for a couple of days to see what happens. If it doesn't soften or improve on its own in that time, then have him checked by your vet.
Thanks for your reply. No he doesn't mind me touching the area. I guess it's a good sign?
No indication of pain or flinching is a good sign. Again, just keep an eye on the area, and take your dog to the vet if it doesn't seem to improve.