That is really weird...interested to see if anyone knows what this is as I'm breeding rats and have never seen anything like that.

Did it go into the freezer like that or are you just noticing it now that you took it out?

My guess (and that's all it is - a guess) is it might be the aftermath of a mite infestation? Does that tail have the same type of wounds on it? Let's hope it's not mites as that would be one heck of a treatment on a whole colony, especially a huge one.