Skinks
The tail curling, no. You should take your dog to the vet about the non-growing hair.
Regeneration
The yellow thing is probably the new tail growing back in its place. i wouldn't worry. also, when the new tail grows back, it will be ugly and not as good looking as the previous one was.
amy cresswell has one on the back of her head, its like a dogs tail growing on the head!!
To keep them warm from ruff winters and also safe from other animals.
Probably not, the usual causes of iguanas tails turning black is because the tank is not warm enough, you will most likely have to take it to a vet to have the tail snipped.
southern colonies
Harsh winters and a short growing season
Tail?
Yes. My Betta's whole tail ripped off and it is growing back. Just put a couple drops off stress coat into the water.
Harsh winters and a short growing season and diseases :(