Prequels
The interquel novel Paul of Dune (2008) by Brian Herbert and Kevin J. Anderson establishes that the first known inhabitants of Arrakis had been the Muadru, who introduced the sandworms to the planet. They had settlements all over the galaxy which suddenly disappeared; the Zensunni Wanderers later came, ultimately becoming the Fremen. In the novel Paul notes, "There appears to be a linguistic connection between the Fremen and the Muadru."
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