Should I be worried?

Our eighteen pound tomcat has some kind of growth on top of his tail, right at the base. It's the same width as his tail and I'd say about a quarter of an inch tall. Its soft to the touch, almost squishy, actually, and it's not bugging him at all. He doesn't even seem to know its there. I'm just curious what it might be, since as far as I know none of our other cats have ever gotten something like this and its the first time I've seen something like it.

Edit: pretty sure I overestimated on the size. He won't stay still long enough for me to get a good look at it.