I hope he'll be okay until tomorrow morning without it getting worse or developing some impact on his ability to breathe and swallow (which is what worries me, I mean, it is really right on his throat), and that maybe it is an abscess that can be drained even by the emergency vet or something relatively uncomplicated. I really think I would have noticed it before now if it had been growing like cancer, which can be fast in rats, but not just from nothing to this large, and I do scritch his throat pretty much every day.

He's just already elderly and has health problems with an antibiotic resistant lung infection, so it's not like he can bounce back from stuff like a young one.