I had a dog who wagged her tail so hard against things! I'm wondering now, looking back, if it was happy tail. She is long gone now, she passed a way a few years ago. But I remember that one time she dented a metal screen door with her tail and was perfectly fine! Or at least, she didn't seem to be hurt. She kept wagging her tail like normal and we didn't see any obvious injuries.
A dog with a tail, who is very happy, does not necessarily have Happy Tail.
Happy Tail refers to a syndrome where the dog self traumatizes the tip of their tail again and again and again.
The typical appearance is an extremely happy which is spraying blood droplets over a wide area from first banging its tail against something, breaking the skin, and then continuing to wag the bleeding appendage all over the place. It can look quite dramatic. The blood is typically spread over a wide area.
While it’s most common on dogs that have whip-like tails, thin skin and thin fur, like the greyhound and SBT, any dog can get it. I would say that many can heal with careful bandaging and management, but chronic cases will sometimes require amputation of the last joint or two of the tail.