Welcome to YT
!!!
Izzy had a growth on her back leg, right near her rear end when I first got her that looked just like that, kind of like a wart but dark. She was 7 months old. Her previous owner had taken her to the vet about it and the vet gave her Animax cream to treat it, but it wasn't going away. Shortly after I got her, I had her spayed and had the growth removed at the same time and it was tested (NOT cancer, thank goodness!). I'd take the baby in and get it looked at and removed and tested, just to be safe.
Good luck!!!