Joining the cloaking snake boss we saw at Comic-Con, is a human-spider hybrid. The ESRB spilled the details about the boss because, well, she (it?) is topless.
One female boss character (a human-spider hybrid) is depicted nude—though without any discernible details (i.e., no nipples or genitalia); during one attack move, she briefly caresses her breasts while straddling on players’ character.
Unfortunately, the ESRB didn’t provide any screenshots, but I’m guessing the spider human hybrid is a mix of these two monsters.
The rating board also says there’s a scene (a contextual kill?) where you see a character "slowly inserting a monster’s body into a rotating fan blade."
Resident Evil 6 is on track to come out on October 2 for PlayStation 3 and Xbox 360 in North America. A PC version is in the pipeline, but it will be released after the console versions.