Stormborn
Explorer
The Cardinal said:the smallest genre? That's probably "Stuffed Animals RPG" - there's a (rather popular though OOP) game here in Germany called PP&P (Plüsch, Power & Plunder) where you play sentient stuffed animals...
Fuzzy Knights RPG is forthcoming (or is it out yet Mossfoot?) and in it you play sentient stuffed animals.
I was going to say Bunnies and Burrows, but obviously got beaten to the punch.
Really, almost anything you can think of has been done at least once, and usually by GURPS. But, do one or two books constitute a "genre" in RPGs? Sure, they might be in somethign else, but specifically an RPG genre? I don't know.
In Watchmen, Allen Moore has pirate comics being the dominant genre, I wonder if in the same world all RPGs are Westerns.