A Squishy Lump On My Dog's Tail?

Okay, my dog is 11 years old, and he's 33 lbs (muscle an fur, he was bred from show dogs, lol.) anyway, He has about two warts on his head, no big deal, but then at the top of his tail, where it connects to his back / , there's a very large lump there, and it's squishy. whenever i touch it, poke it, etc, my dog doesn't seem to mind, so it can't hurt him, he just wags his tail. anyway, as you probably have guessed, my question is about that squishy bump, and what it is. like i said, it doesn't hurt him when i touch it, and it's squishy, like a blister, but it's been there for about 3 years.