I am having a similar problem with my mice. This is the first time I have bred mice. I asked the breeder for 3 females and 1 male and they were all small (barely weaned). One day last week I noticed it looks like two of them have testes. I didn't worry about it. Then yesterday I found one that looked like it had something like the pics from the OP. 'Then this morning there is something hanging out of the backend of the mouse. Its pretty gross. I have separated it from the rest of them. Now I have no idea what to do.