So I took the cats to the vet. As predicted, one was pronounced morbidly obese (the tech was overheard saying, "Oh, my gosh... he has such a tiny head, but he's huge!") and the other pronounced malodorous.
The vet pulled Pepe out of his carrier, gave him the once-over, and when introduced to "It" (as I shall now refer to the cyst because I really, really don't want to gross you out more than once a day), he was rather impressed. The vet took the cat to the back, shaved the long fur around the area in question, cleaned It all out, and brought him back, itching and cranky and with a large, noticeable hole where It had once been.
So it wound up all being good news. Not cancer, not something that's going to recur, hopefully, and nothing bacterial or fungal. We're all back home with two weeks' worth of feline amoxicillin. The sight of the site where It was, though, is now the thing that can induce nausea. This is better than the smell, but does anyone know if you can use Neosporin and self-adhesive bandages on cats? Because that'd make me even happier.