My 16 yo male cat has a wound about half an inc in diameter lateral and a little lower than his anus. I just noticed it yesterday night. He is 100% an indorr cat so he hasn't been running around outdoors or around other cats/animals. He has been acting normal, eating, and does not get agitated when I go in close to examine it. There is no active bleeding, no funny odor, and the exposed tissue is nice and pink/rosy. About a week ago, he was acting funny- not eating, playing, just sleeping all the time. I didnt notice any physical signs then. That episode resolved a few days ago, but the two incidents could be related. I am taking him to the vet later this afternoon, but I just wanted to get an idea of what it was.
Sounds like an anal sac abscess formed and ruptured. It was probably forming when he wasn't feeling well and once it ruptured he began feeling better again. A vet visit is in order and having his anal sacs checked regularly in the future would be a good idea.