It could mean a variety of things, from a bug bite to parasites to infection to auto-immune disease to cancer.
I would start with a trip to the veterinarian for an examination - local inflammation is pretty easy for a veterinarian to spot, as are fleas, ticks and mites. All of these can be resolved in a few weeks with the appropriate medication.
If it appears the problem is an underlying nerve irritation issue, or is trauma, this may take more work to sort out and get under control.
I would start from tail trauma that you didn't see - a close scrape with something or a small bite, for instance. Double check for tick bites or small areas that are red, thickened and painful-looking.
Most of the causes are going to be treatable by a veterinarian, so a visit to your vet may be a good idea.
If he has lost hair around the top of his tail, and it is short and frizzy, it's because he is rubbing it on something like a tree or fence because it is itchy. This could be caused by worms, or maybe dry skin.
It not really serious,he just maybe had a fight with other dog if you let him outside,or you should feed him more often and give him little bath if its posibal with dog shampoo which can be found in most pet shops ;D And if you are really worried just take him to the vet ^^
I would think from Allergies, I have had a pitbull boxer mix, and another Siberian husky pitbull boxer cross...both seemed to have bad allergies...constantly scratching and one lost a bit of hair on her back and tail.
It could have been pulled off, chewed off. The dog may have the beginnings of mange.
Probably that the dog is sick, or old.
Another Answer
Possibly Mange. Get it to a vet.
tail,fur
Possible infection or canine cushings disease, here is a help site link http://www.vetinfo.com/dencyclopedia/deindex.html
i would take him/her to the vet
A SNOW LEOPARD and a DOG
Boxer
I used Epi-Smoothe on my dog. She has some spots where her fur is falling out
Whiskers, fur, paws and a tail. 4 legs, no arms :-)
its only to much for a dog to shed only if it is losing all its fur then contact or drive to the vet immediately
http://www.boxerforums.com/losing-patches-of-fur-t19467.html
Probably one with fur, a tail and, most likely four legs? I don't know if you do not specify
Could be mange, a potentially dangerous condition, but easily treated by a vet.
If they are losing fur it is most likely do to mites.