Went looking for this last night. Had 2 drop from the trees in the south barrow-downs. I am guessing it is a random drop from those trees. Had other trophy drops as well. Interesting, when I had 2 of these roots, the inventory item said: 2 Dusky Huorn Root Fragments.
Didn't matter what they were called though, because they worked as the trophy for the tailoring crit.