Corrected entry: In this film it is revealed that the Minions were around long before joining with Gru but in the first Despicable Me it is implied that Gru created the Minions himself. In the girls' room a poster can be seen on the wall showing a schematic of the Minions during their final stage of design.
Scott215
21st May 2016
Correction: Given Gru's genius and his need for more minions for him to follow, it is not unlikely that he and Dr. Nefario studied their anatomy (the poster on the wall) and figured out a way to duplicate or clone the minions to create more of them.
Scott215