giant knot on dogs butt

What is it when a dog gets a big soft knot on their rear the size of a tennis ball?

[peekawho]

It could be an anal gland impaction (but that would be one heck of an anal gland impaction), or something else.  God help you if it is the anal gland.  If it lets loose...lets just say you'll have to move out of your house.  

A giant knot that size on a dogs butt needs the immediate attention of a veterinarian.  The dog has to be uncomfortable.  There is no other solution but to take your dog right in to the vet.

[Jaybay]

peek is right.  This problem definitely needs to be evaluated by a veterinarian.  When you say "rear", that can cover a whole lot of territory.  Are you talking about the anal region or more about the muscular buttocks area?  That makes a big difference!  Like peek said, if that's an anal gland.. whoa!!!  I've never seen one that big, and generally if you can even visually see an impacted anal gland, that's a whole lot of stinky mess to contend with in addition to a lot of pain for the dog.  

How quickly did this lump appear?  Can you tell us more about your dog like breed and age; general activity level; is he an indoor/outdoor dog; how much supervision does he get... More info will get you more help here.  :-)

Also, you can consult Med Help's vet, Dr. Aleda Cheng, free of charge.  To post on that forum, look to the right side of this page under Related Expert Forums and click on Pet Health to post a question to Dr. Cheng.  Please don't waste time speculating though - your dog definitely needs medical help.