A miracle! Praying to the dog god worked because Beau's growth has virtually disappeared. It went from the size of a walnut shell to no bigger than a pea. Last week I took him back to the vet to get a judgment call on whether to go ahead with surgery to remove the growth. I was worried that it might have grown inward, but miraculously it hasn't. She didn't see any urgency to remove it. It's not likely cancerous if it's getting smaller instead of larger.
Considering his age being around 9, I am reluctant to put him through surgery for something that minor, but I'm still checking on it every day to be sure it doesn't get any larger. Fortunately, he doesn't take it too personally when I lift up his tail to examine his nether region. He did when the doc gave him a rectal exam, though. For the first time, Beau didn't seem to like going to the vet. He hid under the bench with his face to the wall. He used to think it was all a grand adventure.