
It's not a new teat. They are there from the start. Sometimes the very first in the row can be very small and undeveloped, but they are definitely there to begin with.
I'd say the pups have started to suckle in a special place, and then kept on doing so, as that part has been swollen and raised. This sometimes happens. I always remove them as soon as I see this, as I suppose it is painful for mum. And a bad bruise and soreness can develop, which you don't want.
Often pups suckle just for comfort, and don't mind if there's milk coming or not.
HTH
Karen