Favorite monster (animal/vermin).
That’s easy. Spiders!
They range from the tiny wall-crawler to the huge and malicious Shelob-class brute, and they are all lethal!
There’s no better way to give the creeps to your players than describing the room they are about to enter as “festooned with web and stinking of death, the floor strewn with bones and sorry remains of unlucky creatures”.