A number of animals/monsters have a howl that can cause fear, but even though I get an intiative check from the howl, I never have to make any kind of save and I've never been feared by the howl.
My shadow mastiff (From shadow conjuration) has never scared a single enemy, even though it has used the howl several times. But maybe i'm just unlucky.
My mastiffs seem to work sometimes and not others, but usually the effect seems to stun opponents, not cause fear.