Lugus

I believe that the original Gaulish form of this name was Lugos, not Lugus. Gaulish resembled Latin much more than any other Gaelic language. Where a lot of Latin words or names usually end in -us, Gaulish words and names (or, their Gaulish equivalents) usually end in -os. Therefore, Lugus is more likely a Latinized form of Lugos. See also the examples illustrated on this page:
http://www.orbilat.com/Encyclopaedia/G/Gaulish_language.html