RAW (rules as written), there’s nothing that says that they can’t. If you think about it, there were plenty of horse-born nomads like the Mongols and Huns. One could argue that they had their share of barbarians. Attacking opponents with the ferocity of a berserker could happen with nomadic people like especially if they were nearly as comfortable on horseback as they were on their feet. I would say, however, that using certain tactics, like making a pass with a lance in the way that Baggi did against the goblin chieftain, wouldn’t work with raging. You can’t attack, withdraw out or reach, and attack again, and call it a rage.