Originally, the planet of Utapau (shortly called Utapo during production, though the name Utapau goes back to the early drafts of Star Wars) wasn't supposed to be inhabited by the creepy aliens of Tion Medon's race. Those tall guys were supposed to live on Mustafar. That's why they sort of look like they belong there. Instead, Utapau was supposed to have a race of lemur-like beings. The concept of the lemur people was dropped early on and the original Mustafarians were moved to Utapau. New folks were created for Mustafar and that was that.
According to The Making of Revenge of the Sith, the concept of the unnamed lemur people was officially dropped some time around January of 2003. Artist Ryan Church said it best: "Ultimately, we have enough furry fighting things." Depending on their final height, they would have undoubtedly been endlessly compared to Ewoks. Perhaps it was a safer bet to not include them. Other than Church's statement, there isn't much more that I can find to explain why Lucas made this decision. The director calls the shots, though.
Carl Jones e-mailed me with a fun bit of trivia. It seems that Terryl Whitlatch, creature artist extraordinaire for EPISODE I, wrote a book called The Katurran Odyssey which features - you guessed it - a race of Lemur people. George Lucas even helped promote it with some textual raving inside. It makes you wonder if Terryl was the original inspiration for the Lemur people. Perhaps it was a discarded concept from EPISODE I? Somehow, I don't think so but you never know.
Interesting enough, the book contains a blurb from Dinotopia author, James Gurney. I remember something from around the time EPISODE I was out where Lucas' team was accused of doing some heavy lifting from the designs in Dinotopia. I think the Naboo sets were the ones mentioned with the little green domed buildings and the plazas. Reaching back into my memory, I seem to recall reading a bit saying Lucas even called Gurney to make sure he wasn't too angry at the coincidences, but I can't confirm that's what the conversation was about. Anyhow, thanks to Carl for that little bit of trivia.
So the lemurs never made it into EPISODE III, but there was plenty of concept art created which you can find below if you want to get an idea of what might have been.