My neutered male 6.5 year old GSD/Husky/Lab developed this growth on his belly a couple weeks ago. It doesn't seem to bother him much. Any clue what it is? One source suggested it could be a mast cell tumor.
As mentioned in the comment, this is your dog's nipple; if you look close you will find more.
It is totally normal; all mammals have them (both genders). We have had this question here before for other animals.
Be aware of similar looking growths on your pet and contact your vet if you are not able to make a positive identification of what you see.
Nipples have different shapes and colors, even in the same breed of animals, and their numbers might vary even within the same group of babies.