Iconic mobs from earlier versions of FF such as worms can easily be reincarnated as NMs and classified as voidkin.

Bloodshot oozing eyes reminds me of some kid with conjuctivitis... Which is just as terrifying as anything else in this world. (Point thy oozing gaze elsewhere, foul creature!) I think Dodore, with it's creepy cloak of fog and scary Mor Dhona theme fits as an intense mob.

Having the music change around the presence of certain mobs can add to the creepy feature. In FFVII one loud toll of a single bell told you everything you needed to know. Right now it may not be possible, but with the right presentation, even a rabid marmot can be intense.