A Roly Rider is an enemy in 600 A.D. in Chrono Trigger. They are essentially a Green Imp riding a Roly.
Crono encounters Roly Riders while traveling through Guardia Forest on his way to Guardia Castle.
While traveling to and from Guardia Castle and the Cathedral, Crono and his party encounter more Roly Riders in the Forest. They also meet some in Truce Canyon on their way back to the Time Gate.
More Roly Riders are encountered in Truce Canyon and Guardia Forest while Crono and his party pass through on their way to hunt down Magus.
After Crono's party acquires the broken parts of the Masamune, they head back to the Time Gate in the Canyon and battle more Roly Riders along the way.
They encounter Roly Riders in the Canyon again on their way to repair the Masamune and recruit Frog.