I've been thinking while looking at the DD wiki page....I got curious on what primal fear means....not just the power-up but the overall meaning of what it actually means. 

I decided to look up what the definition of primal fear means and it turns out that it's part of the human psyche and how it's an innate fear that's programmed into our brains that's our ancestral need for survival. 

Primal fear is usually common fears of death, snakes, spiders, heights, darkness and isolation.

Now to look at this from a monster's perspective....when Doug uses Primal Fear on one, they become stunned for a short while. 

This could mean that fight, flight, freeze or fawn is triggered when it's effectively used. 

Think about it....when the power is used, the monster is frozen in their tracks along with being in a vulnerable state of shock. 

It could be that their response is to freeze due to how Malak made them the monsters we know today. 

I assumed that when primal fear is used, it makes them see their worst nightmares along with being paralyzed with fear....but then I realize that freezing up must be their response to how they see the mortal as their darkest fears. Again, going back to how they deal with fight, flight, freeze or fawn.

When Bierce tells Doug to convince the Murder Monkey that "you're the bigger monster", we can only assume that Primal Fear is another way to scare the monsters because looking at it from the monsters' perspective, the Primal Fear power must make them see Doug as their worst nightmares and sends them to a freeze response. 

Although, there are some monsters that may sound like they're fine after their bodies are stuck in the freeze response but that may be a coping mechanism to not sound intimidated. 

However, Primal Fear doesn't work on Malak mainly because he doesn't really have any darkest fears that he has. The same goes for Bierce even though she has regrets of what she did in her past. 

The bosses don't really have this kind of response either. 

It could be that they have a different response when the Primal Fear is used. 

They may go into a fight response as if nothing happened. 

I don't really know, I'm just using what I learned from Psychology in high school.