Bereskarns are described as "Bears covered in spikes" and are first seen in the Dalish Elf origin. They are also seen in the mage origin story in the Fade, as a demon of Sloth. If you answer three of the demon's riddles correctly, he will teach the bear form to Mouse.
Bereskarns are bears that have been infected by the blight. Like Blight Wolves, they are a form of ghoul.
A Bereskarn (Named Fade Rifter) appears if you complete the Third Summoning Ritual in the Circle Tower in Summoning Sciences. It drops Charged Mitts.
In addition there is a random encounter while traveling where the warden may encounter a Bereskarn in a deserted field with rubble. It drops no loot.
Trivia
There is a caged Bereskarn on the first floor of the Tower of Ishal in the Return to Ostagar DLC.
The player is unable to interact with this unfortunate creature.