What is the name for this type of anchor bolt with a "leg"?
This is a specialty bolt made up my the manufacturer of the basketball hoop. I suspect you are asking because you need a new one. I doubt if you will be able to find one, but you should be able to make up something very similar that will do the same function. Get a regular bolt, washer and nut, with the right diameter and length. A second washer might be good enough to sandwich in the the square bar to the frame. If not, get a small plate and drill a hole in it for the bolt. The plate will serve the same purpose as the "leg" on the existing bolt.