If that was on the side wall I have seen them grow to a larger than fist size bubble. Then pow the tire blows out. It maybe from a rock bruse, though I don't know how wide the steel belts are under the tread. I guess that is one of the reasons they suggest we replace our tires with in so many years... I'm not sure if its five to seven years?
I would rotate the spare down and make a rope swing with it. Though the tires on my 69 Road Runner are a lot older.