The Green Lantern Corps are widely believed to have started on Oa where they are currently based, but their roots actually lie on a planet called Maltusa and from the Maltusian race. The Maltusians are blue-skinned aliens whose planet is incredibly overpopulated and, when a strange symbiote arrived and attached itself to a group of them, those beings eventually evolved into the highly powerful and intellectual Guardians who went on to leave the planet with their own agendas - that being to begin the formation of the Green Lantern Corps on Oa. Incidentally, Maltusa was also the source of the Green Lantern-related beings the Controllers and the Zamarons. It is believed that some Maltusians fled to Earth and started secret colonies - those beings are believed to have evolved into what Earthlings know as Leprechauns (hence the picture of a DC Leprechaun, in case you were wondering!).