::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.