Most people seem to regard Alan Moore's "Mogo Doesn't Socialize" in #188 in 1985 as the first appearance of Mogo... but I think it could very well have actually been a great deal earlier. All the way back to 1963 and #24. Passing through a system and being beckoned down by a planet that identifies itself as "Green Lantern," then after a bit of trouble communicating convinces Hal to put out/kill the alien fire creature within it theat threatens to destroy the world. He does so and Hal departs, friends with the planet now.
Not unreasonable to conjecture that after Hal parted on good terms, this planet -- Mogo -- was made a member of the Corps and such.
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