The rings used to be generally circles that boxers fought in. As more standard rings were created with ringposts to hold up the ropes, the standard shape became a square, but the name stuck.
Cool, The Only Problem Is Circles Dont Have Corners!
C - Shaquille O'Neal (4 rings) PF - Tim Duncan (4 rings) SF - Larry Bird (3 rings) SG - Michael Jordan (6 rings) PG - Magic Johnson (5 rings)