Got the results back fromt he vet. The tumor was a simple cyst, not cancerous! Floppy was back to himself this morning and his brothers aren't scared of him any more. That vet smell, I guess.
Floppy had teeth cleaning and 1 extraction, plus a cyst removal on the back of his neck yesterday. Since the anesthetic wore off, he's been spending lots of time on the counter and fridge top, presumably to get away from Robocat's rough play.