What Is The Lump At The Base Of Dogs Tail?

Without seeing a photo of the lump or having more information about what it looks like, it is very difficult to give a specific answer to your question. You have given no indication of its size or color so please see a vet if you are at all concerned for your dog's wellbeing.
In older dogs, this could be a sign of a cancerous tumor although without seeing the lump itself and without further information about your dog's age and breed it cannot be confirmed as being cancer.
Lumps like this could also be cysts that can grow on a dog's tail. These do not usually go away by themselves and may require surgery to have them removed.
On the other hand, it could simply be a cut or scratch that has become infected and grown into a lump. As dogs will constantly wag their tails and may scratch and bite at them, cuts in this area can easily become infected. Also, as it is on the tail the scratch is more likely to be brushed against other surfaces which may prevent it from healing.
If your dog is generally behaving normally despite having the lump on the tail, it is unlikely to be anything serious. However, if his appetite is suppressed or he has less energy than usual, take him to a vet as soon as possible to rule out any serious health problems.

Lumps in the dogs can be abscess, hematoma, hives, lipomas, and cancerous growths. Fat deposits are common at the base of tails in dogs, so, it is possible that your dog might be having lipoma. If your dog is scooting or licking his/her anus then it can be anal gland disease.

Anal glands are two sac located on either side of the anus in dogs in 5 and 7 o clock position. These glands secrete a fluid and are expressed during deification. Sometimes due to unknown reasons, anal glands fluid become thick and secretions can not be expressed resulting in to swelling of anus. The impacted anal glands should be expressed by the groomer, owner or vet. Applying finger pressure on the anal glands through fingers can express the anal glands.So, you should take your dog to vet for differential diagnosis and proper treatment.

Well. Not sure about that but my dog has got a lump on his back leg and
it gets bigger. So I would say get it checked and if needed get it
removed because my dog needs his removed now. So I hope that helped
you.My cat had a lump in the lower part of her tail. When we took her
to the Vet they told us that it was once broken and that the previous
owner did nothing about it so the bone did not connect in the right
place. She had to get surgery. They had to break the tail and put the
bone in the correct place. I would say take your dog to the Vet the
lump could be very painful.

It could be that someone might have jerked a know in it, or a cyst, or any other number of things. If it hurts him, then I would take him to the vet about it. Hope this helps, good luck to you and the pup.

Fat deposits usually end up in the base of the tail and is not dangerous and pretty normal.  As for when you push down on his but he yelps - sounds like he needs his anal glands squeezed.  A groomer or vet can do this - ask the groomer if they do it first before making an appt!  It is messy.  Fluid accumulate in the anal glands and sometimes get infected and need to be squeezed out or they form a big boil which has to be lanced.  To keep this from happening again.  Massage your dogs anal glands (one on each side of the anus) monthly - what is does is release the fluids into the body where it is expelled from the body when your dog goes to the bathroom.

My dog does too , its about the size of a grape or bigger , and its right under her tail , it isn't bothering her but we all think its cancer , I think you should take he/she to the vets

It could be a benign tumor or abscessed pore,I recommend still bringing your dog to the vet for extra making sure all is kosher

My dog is 19 yrs. Old and he has this big lump on the top of his tail near the base. It started out sorta small then it got another little bunp on top of the big one. It felt kind of squishy so I stuck it with a needle and now it has a big hole and lots of yuckey stuff comming out and bleeding pretty good. Will it heal if I keep antibiotics on it or should I take him to a vet.

Might be a tumor. did you take him to the vet?

My dog has two identical lumps on each side of her tail, (inline where the tail shaft meets the body) about 1.5 inches long and 0.5 inch wide.  What are they, please?
Thanks

Give him a shower