I've started watching Barbarians on Netflix and I noticed they believed in Wotan (Odin) and I saw they had a different name for Thor. When did they first start believing in Norse mythology and how did it come around?
It's not the case that various German tribes adopted Norse Paganism but rather that the two of them evolved out of a common shared "Proto-Germanic Paganism", in the same way that Scandinavian and German languages share a root proto-German tongue.
It's worth noting that we have very, very few sources on any Germanic pre-Christian religion, even the Scandinavian Paganism so beloved of pop-history. Mostly, the names of gods are known to us through inscriptions on archaeological artefacts and sites, and occasionally place-names, long before they were recorded in writing, and so a certain amount of regional variation is only to be expected.
They [deities either of the Anglo-Saxons or of the Continental Saxons] were not necessarily conflated with those of the Norse mythology easily, as generally assumed. While the thread structure is a bit complicated, /u/Platypuskeeper and my comments in Why is Norse mythology so well-known compared to other contemporaneous Germanic religions? includes some basic information.
The points are as following:
Reference: