If you read my last post carefully you already know this, but the first creature published outside of the original rulebooks was arguably the Mind Flayer. I say arguably because the publication date for the premier issue of The Strategic Review is Spring 1975 and Greyhawk was published in March 1975. At any rate, here are some of the monsters that appeared in the early years of the hobby.
SR 1.1: Mind Flayer. Just Mind Flayer, by the way. The label Illithid didn't show up until the AD&D Monster Manual.
SR 1.2: Roper. Incidentally, the Ranger PC class and the first of what has been jokingly called Gygax's polearm porn appeared in this issue.
SR: 1.3: Yeti, Shambling Mound, Leprechaun, Shrieker, Ghost, Naga, Wind Walker, Piercer, and the Lurker Above. Also included was an article listing some humorous monsters with a satirical bent such as the Droll, Weregamer, and Hippygriff.
SR 1.4: Clay Golem. Greyhawk gave us the flesh, stone, and iron varieties of these monsters. This one rounded out the 4 that went on to be included in AD&D Monster Manual.
SR 1.5: Rakshasa, Slithering Tracker, Trapper. This issue also had the infamous Sturmgeshutz and Sorcery article detailing the battle between fantasy forces and a German armored division.
SR: 2.1: No new monsters. The issue did have the Bard, a new PC class. It was also the first time the 9-point alignment system saw print. Included in this issue was a great deal of errata and corrigenda for Greyhawk which would also be included in later printings of that supplement.
SR 2.2: Catoblepas. Also included was a satirical monster labelled the Denebian Slime Devil.
After issue 2.2, Strategic Review became The Dragon. As one can see, every monster appearing in SR made it into the canon, if not in an OD&D supplement (like the mind flayer) then in AD&D's Monster Manual.