It sounds like a ummmmm hell cant think of name..............kocka had one and me too.......it will come to me in a minute...............or 10!
Cyst, thats what I was earching for. Best get it checked by vet to see if thats what it is cos they can get infected and very painful. Kockas burst in the end and caused loads of problems and it had to be removed at age 18/19! I wanted them to remove it a number of years before but they didnt want to. It was on her back like mine but on a tail could be more of a problem maybe.
I believe there is also something called a fatty lump or something but best to get it checked out.
Same with mine it got so painful and was lanced I think but its started coming back grrrrrrrrrrrrrrrr.