Finding Fenris hair has been a problem for a few players. It hangs from ceilings on either side of altars in Frost Caves. Altars usually have darkwood wolf ornaments with pedestals containing lootable items on top. One can see the knotted braids of Fenris Hair suspended in front of red jute banners. The braids are lootable by pressing the interaction button, but one can also jump on top of the pedestals, then jump and slash with a knife and cut down a few remaining hair suspended from the ceiling. I have also found it hanging on the back wall of alcoves where pedestals, bonepiles, chests & putrid meat are found.