Actually, Caesar and German cavalry had a long history. At a critical point in the Gallic wars, the Celtic Aedui tribe(where Caesar had been getting all his best cavalry and scouts for years) broke away and joined the rebellion against Rome. Caesar, who was the first Roman ever to cross, let alone bridge, the Rhine (he did it twice), immediately replaced the Aedui with German tribesmen who were tough, loyal, and became his favorites. This horrified the Celts, who feared the Germans worse than the Romans. And, as shown in the mini-series ROME, there's no question that Caesar had German auxiliaries with his army from there on in.