The Helmock gets Terrify, not Horrify. It is a telepathy power, and so the argument is that they should get the Primaris, psychic shriek, because 100% of their powers come from that tree, and grey knights get their primaris from powers they did not "generate".
I, myself, am still unconvinced. While there IS a precedent for the situation, there is still no SOLID
RAW granting them the primaris. I could see it being ruled either way,
tbh.
EDIT: Upon further inspection, in the rulebook units already knowing a power is listed under "Generating Psychic Powers", and it then says "otherwise they generate a random power. to randomly generate a power...." Since it always includes the word "random" when describing how to generate random powers, this leads me to believe that there are two ways of "generating" powers. "Automatically", and "Randomly". Using this reasoning, it does seem like the Wraithfighter gets the primaris, Shriek as "Automatic generation" still counts as generation. This is obviously open to different interpretations though.
Upon further further inspection - the wording of terrify has changed to possibly cause TWO morale tests. One at the end of your psychic phase and one at the end of your opponent's psychic phase.
YMDC thread started on this here:
http://www.dakkadakka.com/dakkaforum/posts/list/597517.page