In episode Echo House, Chris Argent relays to Derek Hale he had once encountered, fought a Berserker.

 A few years before the series timeline, Chris learned of a group of people were messing around with a series of rituals. Eventually one particular young man became a Berserker, tapping into the animalistic mindset. Unfortunately, the young man's humanity and personality gradually deteriorated. He killed innocents, tearing their bodies apart, because unlike shapeshifters, Berserkers aren't tempered in the manner the werewolf is by the full moon and Chris was forced to kill the man.