I think he was Morton Bowser Koopa, the King of the Koopas, even though his original name in the Japanese SMB1 manual was Daimaoo Koopa, or "Great Demon King Koopa."
I think he named his kids, and named one of them Morton Jr., and then he had this mid-life crisis, and started going by his middle name Bowser, and named his 8th kid Bowser Jr. This reeked of favoritism, and the 7 earlier kids felt shunned and they ditched his ass. Now it's just Bowser and Bowser Jr. starring in every game together, preteding the 7 cooler siblings don't exist.