Krampus is a pre-Christian or pagan tradition that began somewhere in the depths of the Alps. When Christianity reached the Germanic tribes outside of the Alps, they tried to get rid of Krampus. Here is some video from the 2014 celebration.
However, Christianity had trouble penetrating the mountains, leaving some villages to continue their older practices and still others simply unwilling to forget all of their old customs. After failing to erase Krampus, they did the next best thing and incorporated him instead – so they paired him up with St. Nick and called it a day.