The phrase "original prototype" refers to the very first version or model of something that is created as a starting point for further development or improvement. Full definition
Porsche's original prototypes featured a rear - mounted, air - cooled engine and a rounded, aerodynamic body that's still familiar today. (automobilemag.com)
These days, McMillen might be better known for The Binding of Isaac than Super Meat Boy, a Zelda - inspired roguelike whose original prototype was developed while Refenes was on vacation. (waypoint.vice.com)