From Sanctuary Shard
All Berzerkers are inherently human and derived from one of the many human sub-races. Scholars are strongly divided on if they should even be classified as a sub-race at all, or simply listed under "special abilities noted in humans". We believe that Berzerkers are in fact a separate race that has been infected by or cross-bred with the Alfar. It is pretty commonly believed that the Alfar are responsible for the creation of wolf and bear Lycanthropes. Given that Berzerkers only ever take on wolf, bear or boar shapes the connection seems reasonable.
However, unlike the lycanthropes who can shift painfully at will but are tied to the moon phases, and the Alfar (and their descendants) which can shift easily as they wish, the Berzerker only shifts in battle or during times of stress. Given the manner in which they shift, more like a half breed Hoggur who is "hulking out", some propose that it is the Jotunn, not the Alfar, who are responsible for the unusual trait.
The Berzerker trait seems to show up most commonly among the Cimmerian, Nemed, and Rohirrim with the largest percentage being in Midrvegr. How the trait is passed on and what causes it to develop is unknown. Most Midrvegr shamans swear by a type of man-hood or woman-hood ceremony which is intended to awaken the trait. Berserkr are bear shapeshifters, Úlfhéðnar are wolf shapeshifters, and Svinfylking, which are exceptional rare, are boar shifters.