Rothar was a Fremen and the first wormrider.
In 7265 AG Rothar was in a party investigating a new spicesand when a large Sandworm appeared nearby. His peers scrambled to the safety of a nearby outcropping of rocks before the worm drew too near. Rothar, evidently too stunned by the worm's arrival to move, found himself only centimeters from the creature's side as it rose from the sand; he seized the leading edge of one of the worm's ring segments and held on tightly, perhaps in an attempt to avoid the worm's flashing teeth.
The sandworm rolled quickly, raising the segment the young Fremen had opened high above the surface of the sand (thereby putting Rothar on top of its body, as well). It then sped off into the desert, with Rothar as passenger.
Refined techniques were discovered with the price of many lives of the earliest practitioners who were killed in the attempt; but within another generation the Fremen's means of travel was firmly established.