Creatures that have the ability to take on many different forms. Including human beings.. Maximum output with some simple thoughts...
Berserkers were a group of Norse warriors. They are human, but in battle entered into a nearly uncontrollable, trance-like fury, and transformed into wolves, bears, and wild bulls. This enabled the men to fight more effectively. Their name would give rise to the English word berserk. Berserkers were said to wear the pelts of bears and wolves as they entered battle and could make the full transformation as they felt necessary. They are characterized as having bloodshot eyes, incredible strength, and endurance.
Various Scandinavian kings used berserkers as part of their army or as hired men and royal bodyguards. Similar behavior is described in the Iliad, in which warriors are possessed by Gods and given the power to exhibit superhuman abilities.