You re simply arguing from ignorance quite a lot is known about the ancient Celtic languages and how they worked from the science of historical linguistics Like many Indo European languages it had inflectional endings similar to Latin masculine nouns usually ended in os but there were some in us as well The form of his name is in any case derived not so much from Roman writers but from his own coins I ll find a reference once I have a chance to consult a few books but it shouldn t be hard to find