Bad Guys
These were some very early drawings in the Mario Brothers reimagining.  The boos there were modeled off of the dead in Lord of the Rings.  I was watching the special features and saw little green boos surrounding Mario and Luigi.  The Goombas were a little harder.  I read somewhere that the Goombas were members of the Mushroom Kingdom whose allegiance were not of the Princess.  They were rebels against Toadstool's Mushroom Kingdom and loyal to Bowser.  Later I refined them to be a race of Mushrooms that had been subjugated by the past rulers of the Kingdom, a fact which Bowser used to bring them under his control.