Oh. Crud.
That’s a really-not-a-wart next to Merlin’s tail nub, and just about the same size. (of course, his tail nub is criminally short)
Photo sent to vet with distressed email. I don’t know if this is “looks harmless,” or if it’s “do a needle aspirate and have a look,” or if it’s “rush to get it removed and send to pathology.”