I don't imagine that they start showing up at the beginning as they are a [censored] to deal with. I had a Mole Rat attack today and it wiped out like 10 of my Dwellers(I had 31 total) so I'm just wondering what causes them to start showing up. Is it time? How many Dwellers you have? The level of your highest character? I'd like to know so that I can work around it and not start running into Death Claws which I'll have trouble dealing with even more.