A big chunk of the Viking Age coincided with paganism among the Vikings, and during these centuries, the berserkir or berserkers were seen as humans who possessed supernatural powers by the blessing of Odin himself. In that regard, much had been said about their so-called berserk fury which allowed such men to forgo pain and demonstrate fanatical levels of strength, like killing well-armored enemies in just a single stroke.
Literary evidences do point to such cases of lycanthropy – like in the example of the Volsunga Saga where Sigmund wears wolf skins, howls when aggravated, and even goes on to use the speech of wolves.
Other possibilities of going berserk might have entailed hereditary conditions and even epileptic seizures. In any case, berserkers did project an aura of awe and fear even during Viking times – as is evident from their frequent postings as high level bodyguards of pagan Viking chieftains (as described in Hrafnsmal and Harald Fairhair’s Saga).
Read more on: Hexapolis – “10 incredible facts about Vikings you may have missed out on” (external link)
Sources: The Vikings by Ian Heath (Osprey Publishing) / On Going Berserk: A Neurochemical Inquiry by Fabing, Howard D. (published in Scientific Monthly) / Erika Hagelberg, evolutionary biologist at the University of Oslo in Norway