I noticed, that after I had killed an aurochs, and retreated to get more ammo (losing aggro), I could harvest all the remaining aurochsen hairs without them aggroing me, when I returned.
So either you need to kill the first of the herd, or probably, just aggroing and then de-aggroing will do the trick.
You can then harvest all the herd's hairs without any danger.
Not sure if this belongs in the bug section, but it does seem somewhat exploity, seeing how much LP you get for the hairs.