Onderon's Beast Riders were originally criminals from the great walled city of Iziz. Eons ago, when the only form of civilization on Onderon was in the form of primitive tribes, the closest of the moons called Dxun would briefly touch its atmosphere to that of Onderon's which would allow the drexl creatures to migrate over onto the new world. The tribal civilizations had to adapt in order to survive the aerial predators, and soon developed methods of killing and fighting off the beasts. This allowed them room to grow and prosper, and in time they were able to unite and create a large city with huge walls around it, in order to better protect themselves from the beasts. Over the centuries, the city and the number of inhabitants it could hold grew until it was able to house millions of people and called itself Iziz. According to Jedi Master Arca Jeth, "Eventually the city covered an area a thousand miles square and reached several miles into the planet's crust. Everything behind those walls--indeed, the rest of the planet--was a savage outland where primitive violence ruled."
The rulers of Iziz saw fit to throw lawbreakers outside of the city and into the wilderness, which meant certain death for hose exiled. However, over time, these lawbreakers learned how to survive in the wilds and set up settlements. They even began to take control and tame the flying beasts as their mounts, and soon warlords established themselves and raised great armies. With these armies, the warlords led attacks on the walled city to get their revenge, but none of them were ever able to break Iziz's defenses. The walled city's technology was too great, and in time they kept updating and their defenses so that no number of invading forces were able to overthrow the city. None of this discouraged the Beast Riders, though, as they kept warring with Iziz again and again.