Germanic cavalry was still staffed with the same horses the Celtic Gauls used, though. The only difference was that caesar viewed the gauls as more barbarian than the celts and therefore were more battle ready. Not to mention, he had a particular interest in stressing that he didn’t use gallic help in the gaul campaign when this wasn’t entirely true, and even lists false differences between the gauls and germans (like saying that the germans had no gods and worshipped elements - Tacitus’s later account sees this as false.)
I’ve already stressed though, that the reason there isn’t a Germanian faction is because of the civ quota limitations. They already exist in the anthology of this mod, but I couldn’t justify adding them here.