Dog rectum question

Sorry this is weird and gross but I’m concerned. So my dog has a large lump on his butt, it just developed within the last hour. He’s not as energetic as he normally is and keeps licking it but otherwise seems okay. Still getting up and walking around but it does seem to bother him when he walks. We’re taking him to the vet tomorrow but I was wondering if anyone knew what it was. Googles telling me all kinds of different things.