In my article http://celto-germanic.blogspot.co.uk/2014/11/fjorgynn-early-term-for-thunaraz.htm
I drew attention to the existence of an early Germanic Thunder God who predates *Thunaraz. Linguistically it does appear that Fjorgynn is more closely related to *Perkunos along with the Baltic and Slavic Thunder Gods. Conceptually though *Thunaraz is identical with His equivalent in the Baltic and Slavic pantheons and we would expect this to be the case as they are all localised variations of the same Aryan deity.
Interestingly though Brian Branston in his The Lost Gods of England takes the view that Thunar has His origins in the Rhineland as a result of close connections with the Celts:
"Thunor means 'thunder'. The god was christened (if the verb is permissable) in the lower Rhineland although one could say that he was born there. It was at a time when Saxons and Celts were rubbing shoulders: they traded goods, they traded ideas and they traded gods. The name Thunor I take to come from the second element of Celtic Jupiter Tanarus, the 'Thundering Jupiter' and it must have been adopted into a Saxon dialect during the period before the North West European Sound Shift, that is, before A.D. 1."
However Branston qualifies this observation by stating:
"The early development of Thunor seems to have been as follows. As a weather god he can trace his lineage back to Indo-European times: apart from all the North West European tribes having a weather god, others of the Indo-European complex such as Hindus and Hittites have weather gods with strikingly similar attributes."
Branston's rather cumbersome term North West European is intended to be his replacement for the word Germanic or Teutonic as he feels that they have "an undesireable emotional colouring." Branston wrote this book in 1957. Thankfully this piece of lilly livered political correctness didn't catch on!
So Branston admits that the Germanic peoples can trace their Thunder God's lineage "back to Indo-European times" but the genesis of the name may he feels may be attributed to the Celts. On page 111 of his book he introduces the deity Fiorgynn:
"As I have said, the cult of the weather god under the name of Thunor began in the Saxon lands of the lower Rhine coterminus with the country of the Celts. From small beginnings perhaps, it spread among most of the North West European tribes. Of course, there were other manifestations of the Indo-European weather god still alongside Thunor in Europe. The eastern branch of the North West Europeans had such a god called Fiorgynn whose name suggests that he was kith and kin to the Lithuanian Perkunas and ultimately to the Hindu Parjanya. Fiorgynn, like many other similar local deities, must have been ousted by Thunor."
The names Taranis and Thunor are terms for thunder and this is what marks these two deities out as being different from their Baltic and Slavic cousins. The Lithuanian Perkunas, the Latvian Perkons, the Prussian Perkonis, the Russian Pyerun, the Czech Perun are all descended from the PIE *Perkwunos/*Perkunos and the first element in their name Per has the meaning of oak, rock or mountain in Proto-Indo-European. All these concepts are intimately linked to the Germanic, Baltic and Slavic Thunder Gods. Thus we are left with the idea at a certain point in prehistory there were two Thunder Gods residing side by side amongst the Teutonic peoples, Fiorgynn who is perhaps the elder and the younger Thunaraz who usurped the other.